It can feel very natural, when seeing multiple people, if you have a really nice experience (try a good restaurant that turns out to be GREAT; see a particular art exhibit that's in town for only a few weeks that moves you a huge amount; do a holiday tradition you love [that movie you watch every year! making treats and eating them together by a fire!]) to want to repeat them with all your partners. "Hey I found this AMAZING new restaurant, let's go together!" can be fun - and many people truly don't care where or how you found the amazing new restaurant. But if consistently the "finding" or first dibs on the adventure happens with one partner and another is getting the "repeats" - or you aren't planning different adventures and outings to account for different partners being different people at all... it will likely become a problem over time.
In general, finding ways to honor the differences in your relationships and between your partners can be just as important as the ever-elusive "making things fair." Acknowledging that people are individuals and that you are a multifaceted person who interacts with each of them in your own way (and that not every single experience needs to be duplicated, an equal percentage of them with each partner first) will go a long way to calming an obsession with comparison.
I say this as someone who struggles with comparison in my relationships. The adage that comparison is the thief of joy is definitely true - and we need to be conscious of the very typical tendency (because we're all only human!) of our partners to compare some, just as we do with our metas even when we try not to. It's our reactions to this (and our honesty with our partners) that we have control of, as spoke partners, and as hinges it's asking questions to have a sense of whether something is particularly important and then acting based on that information. So, if our partner tells us repeating restaurants and who is first is no big deal, GREAT, we never have to worry about that as hinges. But, if they say "experience dates" like cooking classes or exhibits that are only temporary ARE a big deal, keep that in mind; and checking in about holiday-style events because it tends to be a sensitive time of year.
I had a partner ask about wanting to do a holiday tradition/holiday movie night with me the first year we were together (early 2010s) and I'd suggested watching Love, Actually because he'd never seen it and making cookies. About a week before we'd planned to do this, my meta and I were texting and she mentioned that THEY'D just watched that movie the night before. I was not pleased about this because I thought we had already set our plan; he said that she said it was her FAVORITE movie and put it on and he didn't think about it being the same until it was already rolling and then decided he'd just watch it again anyway. We ended up picking another movie to watch, and it wasn't a big problem, but I didn't like that I heard it from my meta, not my partner; I didn't like that a spontaneous selection made the plan in advance to experience something new into a repeat experience; and we had a conversation about either making less specific plans if he was going to ignore them or checking in about whether they mattered. These are the kinds of conflicts that often get made bigger by one partner going "why are you so mad about <specific thing>?" and the other partner saying "it's not about <thing> it's about you not caring about it!" Taking the time to ask about the differences, plan a different date, or make sure that there are different nicknames, different activites, or that it doesn't feel like relationships are interchangeable can make an enormous impact on the security of your relationships.