So buying a new boat that has garmin kraken trolling motor and a simrad nss evo3 mounted at the console. My plan is to put a Lowrance pro 12 on the bow of the boat so it communicates with the simrad. What's the best transducer to use on the bow and best way to mount it to the trolling motor?

The Lowrance Pro 12 is best suited and was basically designed for the Active Imaging HD3in1 Transducer. You are going to be limited to having to use a aftermarket clamp to mount to the kraken shaft.
Any reason you didn't match your Garmin Kraken to a Garmin unit on your console and bow? That troll motor has lots of features that will network with the Garmin units that won't to the Simrad/Lowrance units.
If you go with all Garmin or all Lowrance/Simrad. you could at least network everything together.

So the boat came with simrad 12 at console and garmin kraken. So instead of me removing either on of these I am trying to just add what will work best with what's on boat. Didn't figure I need to spend the extra money for another simrad on bow so that's why i went with the pro 12 so it would comminicate with the simrad.

Just not sure on how to mount the pro 12 to the kraken

As far as I'm aware Lowrance only makes three transducers that work with the Pro HDS. The 3 in 1-which has downscan, side scan and active target. The 2 in 1 which has downscan and side scan and the transom mount downscan.
Problem is if you mount the 3-1 or 2-1 to the troll motor you basically can't really use the side scan or active target as you will be rotating the beams as you turn your TM….only the downscan mode will operate correctly. ….but perhaps that's all you really want?
You might want to consider an aftermarket shaft that can just hold the transducers but it's just another shaft to mount on the bow of the boat.
Perhaps your dealer can find a transducer that will work for you?

Don't really get why your dealer/manufacturer rigged the boat with a Garmin Kraken and Simrad/Lowrance electronics. Should have either done a Garmin Kraken and Garmin electronics or a Lowrance/Simrad Recon TM with Lowrance/Simrad electronics to make a one network boat.

How do they have your Simrad wired up now?
Did they install a through hull transducer or hang a transom mount transducer or both?
I would think they should have done both so your through hull transducer would give you a decent image while the boat is on plane?

I have Kraken/Garmin on three boats.

You will regret not being able to network your units and use the TM-56UHD transducer built into the Kraken.

Being able to tap your waypoint and have the trolling motor take you there is just as much a game changer as spot lock.

Consider selling Simrad and installing Garmins for console and bow.

I would say the same….but they probably flush mounted that Simrad in his console and you and I both know that non of the electronics companies use the same dimension hole.
So he will either have to cut a larger hole if he has the room…..or make a custom trim plate to make the Garmin work.
That's also why I asked him about the Simrad transducers……if they glassed one in or transom mount or both.
He may be stuck with the Simrad unless he goes to a lot of expense and trouble at this point.

Still worth it.

Bigger Garmin.... is always the answer. Says my Garmin 1643 xsv....