Baby seat will fit. All 3er coupes have harness for it on the backseats. It's just going to be painful each time. (and will leave marks on the back seats).
Get nice recaro baby seats or euro spec BMW baby seats to match the theme right.
If you're leasing your M3 coupe, you maybe able to find lease trade with M3 sedan owners.
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PAST: F80 M3 ZCP | 911.1 C2S | F10 M5 ZCP | E70 X5M | E90 M3 | E46 M3 | E36 M3
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