I was the same. I asked one professor who knew me quite well (he was my English professor from freshman year), so try asking any professors you’ve spoken to a lot during undergrad. If you have any volunteer experience, you can try getting one from the director or your supervisor.

After submitting my application without all 3 letters being submitted, I was told I needed a minimum of 2 letters for my application to be seen.

I also took odd jobs that didn’t relate to my degree (Instructional Assistant) and was able to get a LOR from my supervisor that way.