Presuming they attacked with say 60,000 combat troops and have been engaged for three days now, and on the offensive so they can't really "dig in", they are exposed and apt to come under fire from those who were dug in .... 3,500 is not beyond the realm. Ukraine would have their entire military in the field, plus some civilians, many no doubt with limited training and gear, but still, they could match the Russian numbers pretty easily. Russian forces would have near superiority in the air and in artillery.
Morale? No question on which side that would lie.