isnt there an aftermarket conversion kit to let the glock fire 3 round bursts?
Not that I'm aware of.
There are aftermarket conversions to make a G17 full auto like a G18, but they're restricted to LE or an FFL/SOT with LE demo letter. There's also the guy in Argentina who spammed many gun boards with his ads for a G17 conversion kit, but he got busted a couple of months ago. Besides, it would be very stupid to order an illegal machinegun conversion kit from a foreign country unless you have a real desire to spend some time inside a federal prison (can you say multiple felony counts?).