It seems that your celery worker attempts to spawn a large amount of subprocesses, or at least a number that seems way too high for a 3GB RAM machine. sentry run worker infers this process count from the number of CPUs it detects, try something like sentry run worker -c 1 perhaps.
We do say the minimum requirement is 2.4GB, but I think this might have been measured on a single-core machine or something like that. Might be worth revisiting or adding a disclaimer that RAM usage depends on CPU count, if the above suggestion works.