I haven't ever had more than two "serious" partners at a time. I once had two who were serious emotionally, now I have two where one is deeply emotional (aka my husband), and the other isn't, but I spend the amount of time with that I'd spend with somebody one on one if it did happen to be deeply romantic.
I do have free time, as well as a desire for another relationship that has more of a really good friend aspect to it. I think it should be easily doable, but I wonder if I am fooling myself, and I'd end up not having enough time for my husband, chores, and personal time. I have found I want to be good friends with a partner if I don't see them often, to communicate regularly with them about life and other mundane crap.
Now I don't think of relationships that are inclusive or akin to marriage too much. I am more interested in dating people already in committed relationships for a variety of reasons, and while I could see myself even perhaps for a lifetime, with a second "primary" (live in) partner, my husband isn't particularly interested in this. I do think of lifetime best friends with romantic and sexual connections though.
I think I could handle two romantic relationships and one casual (heart-wise) but regular relationship. I don't happen to think really casual sex would work for me, but do wonder if I could even fit a fourth occasional thing in there if it was a friend with benefits with the stress on FRIENDS. Now I just have to wait for the right friend to come along to test that theory.
I figure there are a some ways to fit this into life if my current second relationship stays somewhat constant, which I don't know about yet.
1. I spend time with my partner's spouse/family friends and vice versa, so they can be a part of my life but I don't have to worry about having 1-2 days every week to spend alone time with them to feel close. If lucky my husband wants to hang out with them too. I get social time with two or more people who are important to me. Everybody profits. A bit problematic because my husband doesn't tend to be buddies with people I like & the person I'm dating regularly now doesn't do the "family poly everybody hangs out" thing. I still keep my fingers crossed this happens in the future though. If super lucky somehow this involves one of my husband's partners and their families too, and board games and zombie killings and good food and wine!
2. See hypothetical 3rd partner once a week or less but have regular email/chat conversation with them to catch up on friendship maintenance.
3. Find somebody who is free weekdays, so I can still spend plenty of time on the other two relationships.
4. See my other two partners 1.68 times a week, have my husband involved in two similarly intensive relationships with schedules that can all be coordinated to make this work, and make sure the 3.64 days of a week I spend with him always ends up being quality time....
Yep, I spend a lot of time thinking about this