1.Mr(Cr2O3)
2.Mr(Al(OH)3)
3. Mr(Mg3(PO4)2

1. Mr(Cr₂O₃) = Ar(Cr)*2 + Ar(O)*3 = 52*2 + 16*3 = 104 + 48 = 152

2. Mr(Al(OH)₃) = Ar(Al) + Al(O)*3 + Al(H)*3 = 27 + 16*3 + 1*3 = 27 + 48 + 3 = 78

3. Mr(Mg₃(PO₄)₂) = Ar(Mg)*3 + Ar(P)*2 + Ar(O)*8 = 24*3 + 31*2 + 16*8 = 262