Question about Bosch Home & Garden Charger AL 36V-20

The designation "3Ah" is incorrect. The charger is compatible with all 36V batteries from Bosch (1.3Ah, 2.0Ah, 2.6Ah, 4.0Ah, 4.5Ah, 6.0Ah).
Incidentally, the designation also does not refer (with an incorrect unit) to the maximum charging current, which is 2 A.

I do not know of a Bosch Li-Ion battery with "+".
However, the charger is compatible with the normal 36v/4Ah battery (green or professional), e.g. this one:

Bosch battery 36V / 4.0Ah Li-Ion (36V, 4Ah)