Can I replace the 5-wire battery with a 3-wire?

Hi,

The question to ask is the single red wire and the single black capable of handling the same amount of electrical current as the four wires did, i.e can two wires do the job of four wires? They should be, as both the batteries have the same specifications, i.e. 3.7V DC 3000mAH but it is always wise to check. Theoretically the two red (black) wires individually should be half the thickness of the single red (black) wires. The wire thickness, not the wire + the insulation, i.e. one wire has to carry the same amount of current as the two wires did before.

If the new single red wire is only as thick as say 1.5 red wires (which it shouldn't be, it should be thicker) then it may work but it will constitute a high fire risk as the wire will heat up because it is carrying too much current and it will either burn away and fail or burn with the consequent results of a fire in the tablet.

Are the two red wire connections on the motherboard directly connected to each other (The same applies to the black wires)?

If they are directly connected to each other then connecting them to one is OK

If they are not directly connected to each other for whatever reason you may have to bridge across between the two connections with the proviso of the single 'bridging' wire having the current carrying capability of the two wires or the new single wire.