Not on the airline you're talking about but on American or Delta (can't remember), I've been able to get away with a wheeled suitcase that fit in the overhead, a heavy jacket (overhead), purse and a back pack. Once I take out the pillow, puzzle book, and my crochet stuff, the backpack and the purse fits under the seat.
I usually don't push it like that but I guess I didn't get to the gate early enough and they wouldn't let me check my suitcase. I had to take my jacket out of it to unexpand it on the plane. That was quite a bit nerve racking!
I bet in general, the less packed the flight is, the more you can get away with in the overhead.