Atypical cells is a very common finding. I agree that 3 months is too soon, given the negative biopsy and young age.
Instead, there are a couple of inexpensive (insurance covered) biochemical tests you can get that may either put your mind at rest or confirm suspicions that a second biopsy may be needed. The two tests are Prostate Health Index (PHI) a blood test, and PCA3, a urine test after prostate massage.
For PHI, you have to visit a special physician who offers it. Call at the link below to get a list of physicians in your area:
Prostate Health Index (PHI)- Allen