I have a hotspot setup with 3 different Gateways.. I already have split http traffic to 1 gateway, and all other traffic to another. I want to use the 3rd for certain customers based on their MACID.

How can I set a MACID to goto a certain Gateway?

Use mangle in the firewall. There is an option for MAC ids.

Any examples on how to set this up? Because anything I try doesnt work for web traffic.. it still goes out main gateway

Try something similar to the following.

mark the packets with mangle
/ip firewall mangle src-mac-address=00:00:00:00:00:00 action=add-src-to-address-list address-list=XXXXXXXXX

then tell them which one of the addresses on your WAN to use.
/ip firewall nat src-address-list=XXXXXXXXX action=src-nat to-addresses=x.x.x.x