You can either make specific rules:
/ip firewall filter
add chain=forward in-interface=Vlan1 out-interface=Vlan2 action=drop
add chain=forward in-interface=Vlan1 out-interface=Vlan3 action=drop
add chain=forward in-interface=Vlan2 out-interface=Vlan1 action=drop
add chain=forward in-interface=Vlan2 out-interface=Vlan3 action=drop
add chain=forward in-interface=Vlan3 out-interface=Vlan1 action=drop
add chain=forward in-interface=Vlan3 out-interface=Vlan2 action=drop
Or with just one assuming that all three VLANs are supposed to reach some other interface you can do this statefully and go for a default drop policy, which is a good idea anyway. Let's assume that interface connects to the Internet and is called WAN:
/ip firewall filter
add chain=forward connection-state=established action=accept
add chain=forward connection-state=related action=accept
add chain=forward connection-state=invalid action=drop
add chain=forward out-interface=WAN action=accept
add chain=forward action=drop