Remember they are a couple
You may be thinking that having a threesome is a quick way to get laid without having the complication of a relationship. Unfortunately this is not the case and you will have to form a relationship that is similar to a working relationship with them. During your communications with them if you only focus on the woman, getting laid, or your “assets,” there is a good chance you will not be considered. This is due to the fact that he male half of the couple normally acts as a gatekeeper and you won’t get past him if you ignore him. Best bet is to include both in your communications, let him know you respect him, you respect her, and you respect them as a couple. This does not mean you have to give the male half equal attention. Instead it means you need to recognize her partner enters into the equation and by ignoring him you are most likely killing your chances of being considered for a threesome with them.
In any communication be courteous and genuine.
Courtesy, honesty, and genuineness goes a long way with a couple. Trying to be someone who you are not, lying about yourself, or being rude will not get you far. Swearing, using colloquialisms, and being too casual is likely to work to your detriment. If a couple suspects that you are lying, not who you say you are, or disrespectful trust will not be developed. Without developing a level of trust with the couple there is no chance that a threesome will occur.
When responding to online ads respond to the information in the ad
This means read the ad and respond to the ad itself instead of sending them a template response. It also means letting them know how you meet their stated requirements. A generic, template, response or someone who did not read the ad is easy to identify. Most likely this will result in your reply being rejected. If the ad or their reply does not ask for contact information such as contact number or email address then do not provide it.
Be prompt, on time, and let them immediately know if you cannot show
Nothing speaks poorly about your character if you are late or you are no show without letting them. It is not a good idea to be fashionably late as most couples will not wait much beyond their stated meeting time for you to show and most likely will not give you another chance if you do not show. So, it is important that you remain in communication with them before meeting them and let them promptly know of anything that may delay you showing up.
Be clean and smartly dressed
As the cliché goes, you only have one chance to make a good impression. Meeting a couple without bathing and poor hygiene will not impress them. Take the time on your appearance and dress for the environment you will be meeting them. Even if you are going to their home for drinks to see where things go, dress smart for them.
Understand your role
Your role in a two male threesome is to support the husband in providing his wife with a sexual experience that cannot be provided in a monogamous relationship. This means having sex with two guys at once and it is not due to the fact that there is something in the relationship that he cannot provide. In essence you are a ‘toy’ for them and not a replacement.
Follow their lead
This experience is primarily for them and you are there as an added benefit. There may be a time where you may want to help them progress from discussion to the act or enhance the overall experience. In most cases provided you stick within their boundaries you will be alright but it is a good idea to ask before taking a leap.
Ask
If you are unsure about their boundaries, their boundaries appear contradictory, or there is anything else that you have a question about them then make sure to ask. It is better to ask than making the wrong assumption.
Don’t rush
There is a tendency if a couple shows an interest to push the couple towards the act. Let the couple dictate the speed from contact to the act and if they are moving too slow for you then they are not the couple for you.
Don’t be afraid to say no
Remember having a threesome is a two way street. Granted the couple may have most of the control over the situation. However if you are not in agreement or the couple does not meet you needs then you are well within your rights to say no. It might mean that you loose the invite but it will prevent a bad experience.
Threesomes and variations 
I suppose that “outperforming” or somehow emasculating the male host is out of the question. In that case, what’s a guest male to do? Should he tone down his “performance” and let the host male look good? What about when it comes to “measuring up”? Should the guest somehow hide his own size if he happens to be bigger?
Thank you for your reply.
I am not sure if I 100% understand what you mean by ‘outperforming’ or emasculate the host male in regards in a mmf threesome. If a couple is secure in their relationship than a physical quality, such as performance or size is not an issue. Since their relationship is much deeper than sex and having a threesome is an extension of their relationship.
Granted there are couples that may specifically search for a guest male who is more ‘well endowed’ or who can perform well. Under that type of scenario performance or size is not an issue since it is understood that is what the couple is wanting.
For the invited male it is their responsibility they communicate with the couple by asking questions about size, performance, and anything else they feel might be an issue for the couple. By communicating it minimizes the chance any unforeseen issue does not spoil an event that would otherwise be enjoyable for all.