Firing out of Raiders

I know that the units inside can fire at different targets than the vehicle itself, but say I have 3 raiders with lances / warriors inside.

a rhino packed with marines comes within 12 inches.

can i then fire the lances from all 3 raiders (assuming it takes 3 to wreck it) before shooting any of the guys inside?

we've been playing it as you have to resolve one vehicle / models inside at a time at the local store but i think this is wrong.

I've never had that issue come up myself, as normally my target priority follows your stores way, but I believed you can fire the vehicles then the troops inside them.

You can fire every unit in your army that is allowed to fire.
There is nowhere in the rules obligating you to fire certain units in any certain order relative to each other.

So yes, in the above example you could fire;
Raider,
Raider,
Raider,
Passengers,
Passengers,
Passengers.

And be totally legit.
And, yes, it is best to fire the lances until you get the mooks and then gun them down with the passengers.

I will say, that I do use a number of marking methods to showcase to my opponent which unit has or has not fired because I do bounce around a lot during my shooting phase - but as long as you're clear to your opponent about what is firing and make it clear that you aren't illegally firing anything twice it should all be good.

Good tip about the marking Thor I might try that, a lot of times I get, "You already fired that!" and I have to explain there a bros inside.