I have two 3.7 volt 900 mAh LiPo battery, which I connected in series to provide 7.4 volts. But now I want 11.1 volts.
Instead of buying another LiPo battery, am I able to put these two LiPo batteries and a 3.7 volt 900 LI-ON in series to achieve 11.1 volts? Is it safe to power a 1000kv brushless motor, 30a ESC, and an Arduino Nano with this connection?
What I think you needed to ask is "are lithium-polymer and lithium-ion battery cells of the same capacity compatible in a single pack?"
The answer to that is that those are two different names for the same battery chemistry. As long as the discharge current limits match (as well as cell voltage and capacity), you should be good to go within the current limits of the cells.
Where there's a potential for problems is that the two older cells may have some number of charge-discharge cycles already elapsed, while the new cell won't, therefore the discharge curves may no longer match (assuming they did when all the cells were new).
So, while this should be theoretically okay (assuming all the cells can source your 30 A requirement without damage), I'd probably be more inclined to use all new cells for the 3s pack and save the 2s pack for its original application. By sourcing the new cells all together, you can ensure that all their specs match, including their life cycle condition. Lithium cells are relatively cheap compared to an entire drone stack and the potential liability if you have an in-flight battery fire and wind up igniting something on the ground.
