Yup, I have this Ikea mattress for my M3LR and it fits great! It is pretty long and I have to move the front seats up a little bit to get it completely flat, but its really nbd.
It's a hella good mattress for the price btw - I've done quite a bit of car camping on many road trips, this one is the 3rd different mattress I've tried and definitely my favorite so far (other 2 were a Costco and some rando Amazon one).