3.3kV isolated heatsink for TO247 packages
I believe the best solution would be to use aluminum enclosure with heat dissipating ridges and mount devices on thermally conductive insulator. This is quite common approach used by high-power modules, see Roboteq motor controllers, for example.

There are tons of pre-fabricated enclosures on the market, but there are also Chinese companies that can make extruded profile of your choice relatively cheap.

For the insulator look for something like Parker Hannifin product, they actually sell thermal pads already pre-cut for all common device packages.

Also TO247 package already has mounting hole surrounded by dielectric, you probably wouldn't even need an insert tube for mounting hardware.

You haven't specified the devices you are going to use, but if you can find them in Infineon "advanced isolation" package, you will have 2.5kV/60s isolation already, which means thermal pad can be quite thin.