Do I need 3 separate 2×2 MIMO antennas or can I use a 4×4 and 2×2 MIMO antennas?

You can either use 3x 2×2 antennas (2 for 4G and 1 for 5G) or use 1x 2×2 and 1x 4×4 antenna (2×2 for 5G and 4×4 for 4G).

Its advisable to ensure the 5G connection is isolated to its own antenna as the 5G cell may not be located on the same 4G mast. This will allow you to independently align both the 4G and 5G antennas separately according to their signal readings.