E30s don't have the LATCH system (specific metal anchors to attach carseats), so you just have to use the regular seatbelts like you would in any pre-2000 car. I'm partial to the Sunshine Radian 80 b/c both how narrow it is for fitting more seats and all the padding/neck support. It's not cheap, but I can fit three in the back of an E30 sedan. Proof below:

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