I got an old 3s lipo, voltages were 4.08v, 4.04v and 3.75v.
I don't mind bad battery life, its okay if I only use this for 10-20 charge cycles.

Is it safe to connect this battery to ESC + motor at this voltages? (Want to test if ESC and motor is working).
If its plugged into imax b3, will it get balanced?

About to start building my first plane 

As per your charger spec it should balance the the lipo battery. But there is large difference is cell voltage(3.75v to 4.04v) so, battery temperature can rise at charging. if you can control charging current in your charger then you should maintain it from 0.5A to 0.8A.

Keep monitor your battery temp at time of charging & if u have multimeter then check voltage per cell manually during charging.

I have SKYRC Imax B6mini so, I never faced any issue during balance charging & discharging. Even I recovered over-discharged lipo battery that was discharged at 2.8v per cell.

Thanks Anuj300, will soon upgrade to the B6 looks like a much better option!