All the three produce good images. I haven't seen any reviews for S2980 since it was launched. Only one review for L310. So its based on spec...
And for your information, I own SX130 predecessor of SX150. SX150 is a little upgrade from SX130. Increased megapixels, added eyefi probably few more scene modes and dedicated video button. So you can very well rate them both equally as far as image quality is concern.
S2980 is a successor of S2950 which is one of the most popular camera in flickr. S2980 is more like a firmware upgrade than "real" upgrade. Just optical zoom is enabled while recording videos with S2980 (guys correct me if I am wrong).
Both these two cameras does have manual exposure controls. Canon have few things in it's favour low light performance, 2xAA, MF, better video, better shutter range, OIS, price and CHDK against S2980's EVF and focal range.
L310 does have better focal range than the other two but sensor shift, 4 sec long exposure, mono recording, 4xAA and price makes me to suggest you SX150 over the other two models.
So SX150 it is...