Hi mama! Depo is very reliable talaga, pero it's definitely possible na buntis pa rin kahit on it, walang contraceptive na 100% foolproof, and three months without period while on Depo is honestly something worth checking with your OB about kasi it could be the hormone or something else entirely. A few of us mums I know got pregnant on Depo din naman, so don't panic pero please get it checked na para sure, especially kung concerned ka, better safe than sorry talaga!