There's truly no perfect answer to this because you never know when life happens or might get in the way of a perfect MMF experience. I'd always great to find 2 men who have a good bromance rather than 2 total strangers, that eliminates the issue of compatibility.
The worse things that could happen is that one person doesn't get an erection so it ends up being normal 1:1 sex rather than a 3some (like I said life sometimes happens). If you like both guys you can give them another chance for a repeat performance in the hopes that it would "get up" by the next date
The relaxation part depends on how much time all 3 parties have, minimum 2hrs is what I'd plan for worse case scenario if time is of the essence, and yes sometimes the chemistry takes a while to build up. It's advisable to have some sort of group chat before hand via WhatsApp for instance so that you sort of get a sense of the personality of each person, but also to discuss preferences, wishes and comprmises (if any). That way everyone is chilled out (and horned up) about meeting.
Positions doesn't really play a role, unless it's health related, like back pain or arthritis. Just go with the flow and enjoy every bit rather than over think things or trying to have a playbook.
Hope this answers your questions