Window unit question btu/sqft
260 Sq Ft?  Are you sure, lol.  Many variables are missing here.  I have cooled 500 sq ft with an 8k window shaker in northern calif.  So maybe you can tell where you live.  Phoenix? lol.  So let's check the units.  If you can measure the air coming out of the unit with a thermometer that would be the first clue.  It should be in the low 40s or high 30s.  Three yrs old is not very ole.  Second is insulation and air gaps.  Get those two out of the equation and you should be cool as a cucumber.