@SummersBreeze Frankly, I don't think you have a hope in hell. I've been exactly where you are, and my XW wouldn't have even given the thought of a threesome the time of day - she wouldn't even snog me ffs!
I would never countenance doing anything beghind your partners back.
Forgive me, but I'm about to ask a lot of questions...
How long have you been in a sexless relationship? (I took 10 years to decide that I'd had enough, and we separated & divorced, and I'm gutted I took so long)
Do you love him? Can you see you being with him for the rest of your life? Do you want to change the current situation? Does he? Do you have children?
Are you in a financial position to go it on your own?
I ask all this, because you need to decide what you want, both now and in the future.
If you want to stay with him, and have a fulfilling sex life (the two should not be mutually exclusive!), then you need to work on it. why has the relationship become sexless? Is it his age?, health?, lockdown? Does he know how you feel? Have you discussed the situation? Would he consider councelling?
However, if you feel you can't face another x months / years without sex, or he won't talk, or change, or work on things, then maybe you need to consider leaving him.
Trust me when I say that a single woman has a lot more choice than a single guy approaching 60. In my expereince, Feeld is useless for connecting with people. I'm on most of the online dating apps, including Feeld, and I've had more luck meeting women on things like Bumble.. and that includes Fabswingers.