Should I join the 3 sum?

No harm in trying.
Ask about boundaries and state your boundaries.
Just know when the time has come to walk away.
Do not feel obligated to continue month after month.
How to walk away without guilt?
Simply say "This was fun and now I think it has run it's course."
Leave on good terms.

it kinda seems to me like there's a whole slew of problems in that minefield. you're enough younger than them that any sort of normal social interaction isn't really going to work well - you're just too different, basically a generation apart. it kinda seems like they're basically turning you into a living sex toy. which might be fine with you and you're totally into that. to me it sounds like there's a whole lot to go wrong and very little that can be a benefit for/to you.

be VERY careful
such adventures, often come with un for seen complications

sometimes it is a way to make a couple break up

so be sexy with them,, for a while
play it safe. but stay open to the possibilities
be SURE of what this may mean

Before jumping into bed with them, jump into a lengthy conversation. Good communication about expectations of all and each others boundaries is crucial. Plan the conversation on a night where play will not happen. If you all decide to play later, it is done so with fully informed consent by all.

Ask them what they like and tell them what you like more importantly tell them what you don't like
Then if it's suits you make your decision

Proceed with caution. Have a plan B. Tell someone where you are, and to call police if you dont call them by a certain time.

Give it a chance, might love it

Why not?

It is fully your decision.

If you do it, enjoy and be careful.

It’s fun being the third

if you want to try it you should

God yes...that would be so hot

Think that is for you to decide.

not if you don't know the couple,

If you do it, don’t allow yourself to be photographed

Probably not a good idea.

as a man in your fifties you should fit right in 👏

Threesomes, foursomes and moresomes are the best!!