I am not 100% sure but I think you can. But you need more then just by a third chainwheel.
As you see, the axis and bearings are pretty much made for 2 chainwheels. Now, I don't know your technical prowess but you could get separate parts of all the things you need for the group to become a triple. But I am not sure if it is worth the hassle of just buying another complete group. I myself wouldn't do it at least .
To be honest, it is probably smarter to keep your eye out for a deal right now. On the webshop I usually go to they have a crankset of Shimano M660 from €225,- for €89,-.
I don't know what kind of group you have now but just to show you that at the moment, you can get a lot of parts for a good price. In march you will be screwed again though.
Ok thanks for all the good info. Was thinking about getting a more hardcore DH bike that would work up as well.
Or i might just upgrade the fork and the rear shocker on my Merida One Five O 880.
Does the DH bikes with only 2 chainwheels up front have more teeth on them then the bikes with 3 chainwheels?
If yes, is the transmission equal on bikes with 3 chainwheels vs one that has 2 because it has more teeth on each wheel?
I wouldn't look at adding a third chainwheel (chainring) if I were you. That is going to be a lot of money and heartache for something you can do for cheaper and much easier through either changing your chainwheel gearing on the double you already have and/or changing your cassette gearing.
As an example for changing out the chainwheels...going from a 46/36T rings in front down to 42/32T is going to make your gearing lower across the board through all your shifting. Then all you are left with is removing (or adding) a few links from your chain. If you look up what crankset you have on your bike it should tell you what the BCD for the chainrings are and from there you can just order the size you want from a local bike shop or online. Third chainrings are kind of a farce overall because you can get almost the same gearing range from a 2x setup without the gearing redundancies as a 3x setup.
The other option is to look into a different cassette. The change usually isn't as drastic but it still makes a difference.
Adding a third chainring may result in you having to buy a new bottom bracket with a longer spindle length, possibly a new front derailleur if the hi/low can't be set properly on the one you have, possibly re-dishing the rear wheel because your chainline will be off (so your shifting will always be terrible otherwise), the extra cost of a front derailleur, and probably a few other things I'm not even thinking of right now.
I keep several chainrings for my cyclocross bike and switch them out depending on the environment (hilly/flat/muddy/dry/paved/etc).