Yeah, Cap's 3 movies can most definitely be considered a trilogy. A trilogy simply means a set of 3 movies...telling one story with a beginning, middle, and end is not a requirement.
Two of the most popular "trilogies" are far less connected than Cap's movies. The "Dollars" or "Man with no Name" trilogy ("A Fistful of Dollars", "A Few Dollars More", "The Good, The Bad, And the Ugly") are all standalone movies that are simply spaghetti Westerns starring Clint Eastwood and having the same director, yet are considered a trilogy. The cult favorite "Cornetto" trilogy is even less connected, the only link being the same director, actors, and themes, yet it's often cited as one of the best trilogies out there.
TFA, TWS, and CW are far more eligible than the aforementioned trilogies as they are still the same story with the same characters. Again, a trilogy does not mean a movie with a sequel and threequel. What the TDKT accomplished in telling one beautiful story is commendable, but should not take away from Cap's trilogy.