If I have Mikrotik RouterOS installed in a desktop computer with two Ethernet port, do I need to install another Ethernet PCI adapter to have two WAN and one LAN?
it’s simple - you need as many ports, as you have wires 
I read somewhere that you just need 2 actual Ethernet port using nth method which I don’t know if it work with hotspot
/ ip address
add address=1.1.0.5/24 network=1.1.0.0 broadcast=1.1.0.255 interface=Internal comment="" disabled=no
add address=10.0.0.10/24 network=10.0.0.0 broadcast=10.0.0.255 interface=wan1 comment="" disabled=no
add address=7.7.7.10/24 network=7.7.7.0 broadcast=7.7.7.255 interface=wan2 comment="" disabled=no
/ ip firewall mangle
add chain=prerouting in-interface=Internal connection-state=new nth=2,1 action=mark-connection new-connection-mark=conn1 passthrough=yes comment="" disabled=no
add chain=prerouting in-interface=Internal connection-mark=conn1 action=mark-routing new-routing-mark=conn1 passthrough=no comment="" disabled=no
add chain=prerouting in-interface=Internal connection-state=new action=mark-connection new-connection-mark=conn2 passthrough=yes comment="" disabled=no
add chain=prerouting in-interface=Internal connection-mark=conn2 action=mark-routing new-routing-mark=conn2 passthrough=no comment="" disabled=no
/ ip firewall nat
add chain=srcnat connection-mark=conn1 action=masquerade out-interface=wan1 comment="" disabled=no
add chain=srcnat connection-mark=conn2 action=masquerade out-interface=wan2 comment="" disabled=no
/ ip route
add dst-address=0.0.0.0/0 gateway=10.0.0.138 scope=255 target-scope=10 routing-mark=conn1 comment="" disabled=no
add dst-address=0.0.0.0/0 gateway=7.7.7.7 scope=255 target-scope=10 routing-mark=conn2 comment="" disabled=no