Traditionally I have always used a 25u as my first bag and then a coarser 120 or 160u as a second bag to provide strength and prevent blowouts. And this is what we recommend in our SOPs

Granted blowouts are something that is much less of a concern when practicing vac bag tek (clear tek).

Triple bag I think is excessive and leads to too much loss to the bags.

Recently we have been running 2x 25u so that any bubble that might get trapped on the first bag during filling still gets filtered.

Have not tested the 15u vs 25u to see if it provides an advantage. Would love to hear some experiences on this if anyone has tried it. Perhaps this could be one of the next Hashtek experiments. 

My go to every time is 2 x 25u wrapped in a 160u. I do 40 - 60gram presses all the time and the second 25u bag pretty much eliminates any chance of blowout, even if I get a little too aggressive with the press. I imagine there is a bit of additional loss in comparison to just one 25u bag but this is nothing in comparison to the loss with a blowout. 

Lately I filter everything through 5μm.  I’ve tried lots of combinations and it always gives the cleanest material.  The presses are slower than higher micron bags. You have to hit the final press at high heat to get any trapped THCA to flow. I recommend 275-300°F for the last press.  

One advantage of the finer micron is less rosin loss to the bag. When using a larger micron outer bag the open space fills with rosin that is lost to the bag.