We have 3 ISP connections (BGP based) coming to our redundant pair of MX routers. These routers connect to a WAN Switch and goes to Firewall and goes to core switch cluster. Now I would like to know what is the best practice to handle ISPs in terms of VLANs in the lan side. Is it better to have 3 different Vlans or trunk all ISPs in one VLAN in the wan switch and core switch? How do generally companies handle different routed ISP connections in Lan side? Do they separate them using L2 vlan tags or never mind about it?
From my perspective, in keeping with the KISS principle you should only seek to differentiate those ISP services into your network as far as the devices that need to know the difference.
In other words, if you are using the ISPs interchangeably for generic Internet access, keep it simple on the internal network and just have a default route to the firewall, and let it (or the router beyond it) sort out how to split traffic.
If on the other hand you are using the different ISPs for different traffic types, you may need to "extend" some notion of that differentiation into the internal network through routing, tagging, NAT-ing or whatever.
So getting back to your original question, only create 3 different VLANs if you have a good reason to. Otherwise you're just complicating things without a corresponding benefit.