Originally Posted by jonmanjiro

Out of curiosity, I did a bit of googling. 3x10 isn't possible, but 2x10 might be with your current RD and cassette. Here's how that would work out.
Assuming your bike has a RD-M610-SGS long cage rear derailleur, your RD capacity is 43T. Your 11-40T cassette is using 29T, which leaves you with a remaining 14T. So you could theoretically install an FD and double front chainring with a difference of 14T. Lets say you keep the 53T chainring you already have and add a 39T chainring. That would give you the following gears. As you can see, the top 7 gears with the 39T chainring overlap gears already available with the 53T chainring, so you really only get two to three new gears at the bottom of the gear range. Great if you want to climb mountains fully loaded, but unnecessary otherwise.