Thanks for the question! So I think I’m personally a fan of doing a mixture. Obviously, the 3-section practice tests are going to be more like your LSAT-Flex when you take it. But I’m a big believer in “if you can do a four section test, you can do a three section test.” Overpreparing can’t hurt, so if you have the time, I’d throw some four section tests in there. If you don’t, no big deal. If you do end up throwing those four-section tests in, you could probably just take the average of your two scores and use that one. The sections might have different numbers of questions, so it might be best to think of it as taking the average of how many you got wrong—if you got 2 wrong on one and 4 wrong on another, then average that to getting 3 wrong on whatever the original LSAT-Flex was. Or just use the section that would’ve been included in the LSAT-Flex for that grade, and then look at what you would’ve gotten on an actual test if you’re curious.
Hope this helps! Feel free to ask any other questions that you might have.