That would be iffy. It would be like hooking up 3 AA and a D size battery in series
Youll get more mAh out of the last 3 wired in parallel but the other 3 in series will die first making you extra mAh only 1.5V
It wouldnt be the end of the world and nothing is gonna dramatically explode but i wouldnt recommend it. Give it a shot thou. I may be wrong and you end up discovering the power source for a perpetual energy machine. You wont get anywhere by following rules and warning labels right? Lol