Good on you for having lessons and taking things slowly, definately the way forward!
But I definately would not recommend breaking a 3year old. I think handling the 3 year old would be a good thing to do as it is alot less advanced on the ground and you could learn alot from doing this but I would recommend and experienced rider to break the pony.

Like Sharon_Gurney and Cascara and a few others have said, backing is serious business for the horse and the rider. It is a very uncontrolable position to be in, especially being a beginnner!! If that pony decides enough is enough then there is quite a slim change you will be able to keep toe situation controlled and yes it would severly knock your confidence, as well as the ponies.

This is not meant harshly as you are obviosuly very enthusiastic as to getting back into the saddle but I lost my confidence and regaining it is a big deal. Something tiny could happen which would knock you back to square one! It has taken me 3 years to regain mine though knock backs of dealing with horses which were too much for me.

I now am almost 100% but only because I took it slow.

Good Luck in the future!!