I was taking my bottle cages off to wash the bike and on cage has the bolts really siezed. Rather and run some liquid wrench I tried to torque them out and rounded the 3mm hex nut. Any easy way out besides having to square two sides of the bolt itself and use a set of pliers? I could just leave it but really I would like to be able to take the cage off.

Drill then use a screw extractor and replace the bolt is how I would do it.

Soak in penetrating oil, then remove with vise grips. If that doesn't work, carl_h's approach would undoubtedly work well, but it is more tool intensive.

Cheers

The best remedy depends on the details. For example if the boss is a riv-nut, drill out both the screw and nut and replace it. Otherwise, if you have a Dremel you can make a slot and try turning it out with a screwdriver. Or grabbing i with pliers, but if it's a 3mm hex, odds are it's a button head screw and there won't be much purchase.

For a brazed boss in a steel frame, try drilling it fairly deep with a 3mm drill, keeping it straight to avoid cutting into the threads, and removing with an EZ-out (works half the time), or drilling it through with a 4mm or #19 drill and retapping.

best penetrating oil is Kroil aka Aero Kroil, if you can find it. used in the aircraft maintenance industry. orange spray can, a /good/ autopart store might carry it, a big chain store probably won't. 2nd best is PB Blaster.

I'd probably try to dremel some flats on it so I could get a better grip with those vice grips. but, those typical 'filletted' (dunno how else to describe them) accessory screws that come on most newer frames are pretty much junk grade stainless, I try and replace them with proper M3 hex machine screws.

careful if you drill it out, its pretty easy to destroy the frame insert they thread into, then you got a new problem....

dremel a slot in the head and use a big flathead screwdriver, combine with liquid wrench

yeah, a 3mm allen really does round out too easily; I always replace them with 4-5mm socket heads before they make trouble.
I guess manufacturers use them because of the aesthetics of a low profile bolt head....dumb

I had to use the dremel then crank it around with vice grips at one point, I cannot believe it was in that tight I used grease before I put them in but I guess after time they stick.

I have used this method. Make sure the screwdriver has nice sharp edges that aren't rounded off from wear. I put the screw driver in the slot and tap it gently so that it gets a better bite.

I have a cheap set of Torx wrenches that I use for this very purpose. Take the next size up that will fit, and lightly tap the head of the wrench into the bolt head. Usually gets the job done, but it does have a habit of rounding out the wrench slightly.

xenologer +1

Also use a mixture of automatic tranny oil and acetone. Also +1 on then tapping in on the screw with the screw driver. The reason for that is it tends to "unload" the screw and free it up.