I think your preflop 3-betting range vs the CO is a bit too tight.
Mine(6-max) looks like 88+, T9s+, AJo+, A8s+, A2s-A5s, QTs+, K10s+. Something like that which is about 11% of the hands.
BB vs CO is way tighter 3-bet and mainly for value with some bluffs thrown in there from time to time. But my calling range is wide (depending on raise size, but given a standard 3x).

My second question is this. When I 3 bet out of the blinds what should my raising multiplier of the original raise be. I have been using 3x the original raise or 4x if there is more than one caller of the raise. So if it is 3BB I generally raise it to 9BB or 12BB depending on the number of callers to the raise. Does this sound appropriate?

Your raise size should be adjusted depending on the potsize. Try to create a situation where the person gets just worse than 2 to 1 to make his call. Say he opens to 3BB, pot is now 4.5BB. We raise to 10BB he now has to call 7BB to win 14.5BB so better than 2 to 1. but if we go to 11BB he has to pay 8 to win 15.5 so just worse than 2 to 1.
If there's more players involved it's called a squeeze play. The sizing of a squeeze has to be high to be affective, mainly because there's already a lot in the pot. Guy opens to 3BB, one calls. Pot is now 7.5BB. So we need to go to about 15BB.

The part I need to work on is what happens when I 3 bet and miss the flop. For example:

folds to CO raises who raises 3BB. Hero 3 bets to 9BB with AQo. CO calls.

Flop is K95r

It seems like my c-bets get called way to often when I miss the flop. Then after I check turn due to no help I'm generally folding to a bet. It seems to me like this is a fairly large leak in my part.

This is pretty hard to explain in short and is a huge part of solid poker. First you need to think about what he would call your 3-bet with. Let's say AJo+, A10s+, KQo+, 88+. Just an example.
Then we have to think about how that connects to the board, what we are saying we have and because of that what he will do with those hands.
For our cbet to take it down as a bluff he needs to fold a decent amount of those hands on a K high flop. Is he folding anything better than 1010? Probably not on a standard cbet in position. It's going to take 2 barrels. Is he folding a K? Nope.
Folding A10+? Sure. Most of the time.

So with AQo on that board we only fold out worse (A10-AJ), some splits (AQ) and almost always get called by better (pockets and Kx). So a bad spot to cbet.