[VinnyP]

I've never used one but heard horrible things about it. A 2x convertor is a last resort for me so I imagine a 3X might disappoint. The Sigma 70-300 will almost certainly be better. I can recommend the Nikon 70-300 VR it's not quite at it's best at 300 but still good and the VR works very well. I imagine you could get one for what you could sell your Sigma for plus the cost of the 3x Tcon.