While playing any chord that involves the G, B, and E strings on my acoustic I get a very noticeable buzz. Open, they're fine. I'm pressing down as hard as I can using my fingertips but nothing is changing. Am I doing something fundamentally wrong or is this an issue with the guitar itself? Thanks!
Most likely it is a problem with the frets or set up on the guitar.
One common issue that comes up with guitars that have been played for awhile on the treble strings is that the frets under the thinner strings will develop grooves or divots - (worn spots where the strings come in contact with them). To determine if this is an issue with your guitar, look very closely at the first 3 or 4 frets under the G, B and E strings to see if there is more fret wear there. You may even see a noticeable divot or grove in one or two frets, particularly under the unwound B and E string.
If that is in fact contribution to your buzzing, you may need to have your frets leveled (by having them filed down with the proper tool, crowned and dressed, or if the frets are badly worn, you may need to replace them.
Another possibility is that your action may need to be adjusted. This should be done by a qualified guitar repair technician or luthier. Properly setting the action may involve adjustment of your truss rod, adjustment to your saddle height, adjustment of the nut height or slots, or a combination of the foregoing.
If you have recently switched to a lighter gauge string, this could contribute to buzzing if the diameter of the strings allows it to sit further down in the slots in the nut. Also the lower tension on the lighter strings can cause them to buzz when heavier gauge strings may not.
No matter what the problem is, it should be fairly easy to remedy once you determine the cause and take appropriate action to address it.
It's the guitar. Check whether it buzzes on all frets or only part of them. While you're at it, check the other strings as well at frets you don't usually play. Here you can find a table qualifying the buzzing problems by type.