Yes, you can. In no universe would you travel for 90 days without doing laundry, no matter how big your bag, so really all you need to do is pack for 2 weeks and do laundry periodically.
Your challenges will be seasons (when are you going? Will you need cold weather and warm weather clothing), and activities (in 90 days it is likely you will go to a beach, go hiking, and go to a nice restaurant or theater requiring dressing up in addition to your normal everyday clothes). Think layers and multipurpose. A simple sundress can be worn for daily touring, or with a scarf and jewelry to be appropriate for a dress-up occasion. Put a cardigan and tights with it and it's fine in cooler weather. A t-shirt can be a beach cover up, a hiking top, pajamas, or just an everyday shirt (and wear the cardigan over it when it's cold).
Shoes - you can get away with 2. Comfortable sneakers or other walking shoes for daily that can also be worn for medium hiking, and flats or sandals that dress up but are also comfortable for walking. If you plan on going to a beach or using shared showers, then flip flops would be important too - the small, cheap rubber ones would be fine. If you plan on doing a lot of serious hiking, or going to a lot of fancy places then that will affect your choices, but if those things are a smaller part then you can make do.
Things like toothpaste, shampoo and deodorant will need to be replaced as you go - bring the travel size, use it, toss it when done and buy more. That's easy to do - no need to carry 90 days of shampoo around.
I used to always suggest 2 wheel bags - they are easier to roll on cobblestone and uneven streets. But lately I have switched to a hard-side 4 wheel. I find that even in Europe I am mainly on concrete and sidewalks and can manage the cobblestones when necessary. The hard side bag just seems neater, easier to keep organized, and actually fits more stuff. So I think it's a matter of preference.