The batteries are "dumb" enough. The third pin contains a resistor to signal the capacity.

120 kOhm apparently means 1500mA (I measured BP-5L and BP-4L).
68.6 kOhm for a "720mAh" BL-5C compatible third-party battery.
75 kOhm for Nokia original BL-5C.

Connecting more than one "third pin" together would drop the resistance, which would signal lower capacity, so that's probably not what you want. Probably the easiest is to just have the BSI (third pin) pin of the biggest battery connected and the rest unconnected. The batterymeter would think the battery is smaller than it really is, but that just makes the meter pessimistic, I'd think.

It's VERY IMPORTANT that any Li-Ion batteries you connect in parallel are balanced, BEFORE you connect them together in parallell. This means you need to measure their voltage and make sure they're within 0.05V of eachother, or thereabouts. If you don't do this, power will flow uncontrollably from the stronger batteries to the weaker batteries, potentialy overheating both batteries, which will damage the batteries, and risks an explosion that can damage property and life.