Canon Rebel T3i

[Kombi]

Welcome
I have a T2i and started with the 18-55 and 55-250 like you.
I went with a used 24-105L and have been happy with my choice.
When you first pick up the 24-105 you will think it is huge and heavy- when compared to the lenses you have, but get used to it quickly.

Suggest reading the reviews on this site. the 18-55 is small enough you could carry around with your walk around.
but maybe consider the Canon EF-S 15-85mm f/3.5-5.6 IS or EF-S 17-85mm f/4-5.6 IS Lens
and if you haven't looked at it, here's a link to Bryan's reccomendations
http://www.the-digital-picture.com/C...pose-Lens.aspx

* I had a short list of lenses and chose the best for me based on the first I could get within my budget ($750).
Although still extremely happy with my lens I have learned more about my camera and how I can use the lenses. If I was choosing today I would probably go with the 17-55.