Since you are wanting to allow traffic only from 2 /24 subnets, you want to start by creating an address-list for those 2 networks.
/ip firewall address-list
add address=192.168.1.0/24 list=Allowed_Networks
add address=192.168.2.0/24 list=Allowed_Networks
Next you will need to create the rule(s) to allow the traffic to pass through the router.
/ip firewall filter
add chain=forward dst-port=3389,80,443 protocol=tcp src-address-list=Allowed_Networks comment="Allow Traffic"
You can check that this rule is being used by enabling the log option.
Then add your default deny rule when you are happy.
/ip firewall filter
add action=drop chain=forward src-address-list=Allowed_Networks comment="Default Deny Rule"
These rules wont affect traffic to the router itself, but only control what traffic is passing through it.