I've owned all 3, still have the HK P30. Following is my opinion only.
Sig is a great shooter, but heavy, I liked it, & if only looking for HD, it would be a great choice. Nice trigger, decent ergo, mine was accurate & 100%.
I've tried Glocks more than 1x, I just don't like how they feel, & hate their triggers. I will admit I shoot them very well. If you like your Glock, you'll like another.
HK P30 has best ergo of the 3, accuracy between them I say is same, I much prefer shooting my P30 over the others. Sights back on target is quicker, it's comfortable to shoot, I've never considered selling mine (Light LEM Trigger).
Note: Yes the HK bashing will come soon, it's really what you like, what's comfortable for you, & what you trust.
PS: Have you looked @ HK VP9?
Any of those 3 would make for a great addition. I"m personally an advocate for standardization especially if you are going to swap between platforms often so I'd say Glock...but if you are leaning toward the 17 why not go just a bit further and get the Glock 34. Same size frame as the 17, longer barrel but a lighter slide than the 17. I love my G34 and it is the perfect big brother to my 19.
Kinda leaning towards this...