Looking to get a third unit for my boat with side scan, not wanting to spend a lot due to getting new boat after first of the year.
just going to go with a 7" unit been looking at the Lowrance elite 7 TI with total scan transducer, Hummingbird Helix 7 SI/GPS and the Garmin EchoMAP 73 SV.
Lets get the discussion rolling.

I have the Helix 7 and I am satisfied with it. I have a friend with the Elite 7 and he's satisfied. Don't know much about the new Garmins, but read good reviews. I guess it all comes down to personal preference and price. I know that wasn't much help, was it?

What are the two other units you currently have?
Where are they located and where are the transducers?
Networking capability?
Mirroring capability?
Where are you planning on putting the third unit?

I prefer Garmin

I have the Lowrance Elite 7ti and love it especially with the platinum Navionics Hot Maps. The guys at BPS are very good at demonstrating all the units so if you can get by there it would be worth your time. The touch screen on the Elite 7ti is what sold me. Good luck on your choice.

I have a hds5 on the bow and a elite 7 on the console, I was wanting side scan and was going to put on with a mount side of my console

Garmin

I have Helix 7 units on my boat. Word of advice....

First...Get the HD transducer. The standard one is not nearly good enough.
Second...get an aftermarket bracket to hold them. I almost lost two of them overboard. First bracket broke on heavy chop and was dangling by the cords. I bought a replacement and thought I had a "lemon". The replacement broke just as easily. I had to make some out of metal at work to keep them in the boat.

If you get the "9", it does come with the better transducer and a better bracket. Plus, it's bigger!