How do I get past the turrets at the beginning of test chamber 15, chapter 3

Solution 1:

I actually came up with a different solution myself. I stood by the portal and noticed that despite looking like the bridge is keeping you from using it, you still have half a usable portal. So I walked through and picked up the turret and tossed it back through the portal.

Solution 2:

Simply ignore the four turrets to the right; the bridge is in front of them so they're harmless.

As for the other one: with a portal, shoot a heavy light bridge in front of the single turret (like in the screenshot). Walk to the just-shooted bridge, and re-shoot the portal so the bridge disappears. If you're standing to the left or right of the turret, you can simply pick it up without it noticing you.

Solution 3:

I used a very low tech solution. First, I crouched down by the turrets on the right and grabbed one of the fallen ones. Using that, I knocked over the two that were still standing. I then walked over to the turret on the left and did the same thing. I've no idea if that was the intention, but it worked for me. :)