You can easily get two poles per OpAmp when building active filters; three poles if you are building low-pass or high-pass.
To make an active LPF with an OpAmp with a cutoff at 5Mhz, you would have to use an OpAmp that has a Unity Gain Bandwidth of 50+Mhz, not the sort of thing you likely have laying around.
Since you are starting with a squarewave, how much suppression do you need of the third harmonic? A two-pole filter will only have ~20db suppression of the third harmonic. If you are generating an RF signal which ultimately reaches an antenna, you should have like 60db suppression of the third harmonic, which takes a much more elaborate filter, beit LC or active, more like 6 to 8 poles.