To add to what Shadow said:
First thing out of the box, make sure your mount is PERFECTLY plumb, if not, you're wasting your time, get it plumb.
Make sure, as was pointed out by Shadow, to aim for the 101 first, an easy way to do this, is to use foil and cover the 119 and 110 LNBs, this will insure that you aren't getting the 101 sat on the 119/110 LNBs on accident, leaving the 101 clear, then get the 101 aimed, make sure you can get guide data, check, on your receiver, channel 490 and 491, you should see 13V for 490 and 18V for 491, check your signal levels on the 101, make sure there are a total of 32 transponders (as the 110 and 119 have less), some of the 32 will be at 0, this is normal, once you know you have the 101 in good, remove the foil, if the skew(tilt) is right, they should be there, if not, than your skew needs adjusting, start off your skew at the position your receiver gave you for skew in setup, then, adjust the skew in very small increments, check signals on all sats, if not at least low 90s (I'd go for mid-high 90s if you're a perfectionist), adjust the skew again, wash rinse repeat.
And ALWAYS remember, to remove the power from ALL receivers before dis/connecting coax, as if you accidently touch the coax ends to metal, you can short out an LNB (btw, turning the receivers off is NOT good enough, unplug them).
And of course, there is the possibility that if there is indeed a tree partially in the line of sight, you may not be able to get all 3, in which case, you can find a new location where you can get all 3, or you might want to try a 2nd dish to grab the missing sat, you would then need a switch with a flex port, if I'm not mistaken.
Good luck.
In case you have not seen it, below is the url for the official install of the phase3 dish:
http://www.directv.com/learn/pdf/12429_00_AntennaManual_1D.pdf