I'm 19 yr old 5'10 and of 63 kg (133 lbs). I've started gym now and I'm doing starting strength program 3x a week. Now on remaining 3 days I do cardio ( treadmill, cycling etc) as my trainer told me. My goal is to gain more muscle mass

So is cardio going to be bad for my goal as I already have low weight or should I do something else on rest of my 3 days Any other advice for utilizing my left over days is also appreciated