The irony of that statement is that here in Texas I only own a single pistol and rifle. My uncle in Wisconsin however has an arms room that would make Chuck Norris and Charles Bronson both a little jealous.
I spent some time in the Cheese-state, and a still get chills when thinking about the weapon-rooms and overloaded gun-racks in the pickup trucks.
Apparently those deer are wearing body armour and driving tanks these days, if the recommended firepower to bring one down can be believed..
But back to OP question.
In SR2 this damage simple couldn't be calculated, which is why mixed ammo was disallowed under the system (except for Tracers in Full-Auto)
Even now in SR4, a burst with Standard/Ex/Flechette simple wouldn't work.
Now, if you absolutely wanted a house rule for SR2 on how to deal with a player that did this?
- Ex ammo adds power (reduced by Ballistic
- Flechette ammo adds damage code (Reduced by double-impact if I recall?)
- Regular 9 Standard damage, reduced by Ballistic armour
So, for your 9M 3-round burst, your options could be:
- Make it 12S for the multiple bullets, add +1 power for Ex ammo (13S), +1 code for flechette
(12D) and reduce by Impact x2- Ignore the modified ammo, and consider that Ex and Flechette cancel each other out

ergo standard
12S reduced by BallisticHowever, all this is off the top of my head, as my SR2 books are back on my shelf and unread for a few years now

As a note, I loved SR2, but as a gamer of all versions SR1 - 4, I
really recommend you look at SR4. It's a much more streamlined rules-set, but still captures all juicy Shadowrun goodness

Cheers!