If you don't know yourselves, a bunch of internet strangers are not likely to hit the nail on the head for date ideas. In every possible scenario there's the chance that someone will feel left out at some point during the date. How they perceive that moment (as deliberate malice from the other two vs the other two being swept up in having a good time in the moment) and how they manage that moment (have a meltdown vs advocating for their own needs, or just taking a chill pill for a minute) are what makes a date experience that would normally work for two, work for three. Anything at all.
Example, ice/roller skating.
Two can skate pretty well, One not so much. Two spend time helping One get their balance, everyone together, having fun. In due course, One has a rest and Two go have a smooth, fast skate together. One could look on in envy or One could look on in joy. That's really up to how One is self regulating. If One knows ahead of time that one would find that emotionally confronting, One could say, "no, not skating," or One could do the work to grow out of their old comfort zone and embrace that triad dating sometimes means that Two have a moment together within that date. On a different date, who is One and who are Two could shift given the activity. Next up, 10 pin bowling. Different Two are good, One isn't. Does One sook like a child or look for the fun in the evening regardless of score?
Or maybe plan ahead so that each person takes turns in choosing the activities of that date and the other two do that regardless, and self regulate accordingly. One plans the date for Two rather than Three planning together. If can be nice to be "taken out" rather than being part of the planning process and if One is doing the planning rather than Two, that helps mitigate a sense of couples privilege. Sure, the activity might not necessarily be one that everyone is absolutely smitten with up front, but Two may get to learn something about One or about themselves as individuals. Gallery visit date? One plans, Another discovers there is a type of art they actually really click with. Third person is more keen on the dinner later. Cool, just don't sulk when One and Another have a bonding moment over some art.
Hardest of all, do something that no-one has done before. Still can't plan for every possibility that someone feels something hard. Dealing with hard feelings is part of relationships, poly or mono.
Triad dating requires maturity and self management skills, and sometimes, to develop those skills, one must be put in a position to use them.
So pick any activity at all and make it work for Three, fully expecting that there are likely to be moments of Two/One during the date.