Hi, I have worked extensively with both the MSB Link DAC and P.T. P-3A DAC. I have not heard the Bel Canto DAC 1 so I can't comment.
I have done extensive mods to both Link DACs and the P-3A for which I also offer commercial mods. They are both great units. The stock P-3A easily betters the Link DAC IMHO. The modified units are closer in comparison. The Link DAC with mods is FAR superior to the stock unit and is much smoother with greatly enhanced/expanded soundstage. The P-3A with mods is also much smoother and richer sounding with even greater extension and detail.
When I compare the two units with mods head-head (no upsampling for Link DAC) I prefer the P-3A with signature mods due to its greater resolution and detail. The Link DAC sounds a bit smoother perhaps due to a lack of resolution. The modified Link DAC sounds VERY good, but compared to the modified P-3A, it seems to lack resolution.
They are both excellent units and are also at different price points. The Link DAC with mods is around $450 used and $600 new and the P-3A with mods is around $880 used and $999 new. I feel that they represent different levels of performance for different budgets.
I hope this helps,