The Square Roll isn't really a roll.
The fingers don't "roll" across the strings; they just go back and forth.
This is often the most basic picking anyone will do.
You can actually do it with just two fingers instead of three: TITI or TMTM
The basic picking pattern is either a T I T M or a T M T I
Here are the same rolls on the same strings but using a chord (either C or D7)
You can also drop a note, usually the second one, to get a kind of frailing/guitar folkstrum sound.
The Square Roll is often the first "roll" you would use when trying to figure out the melody to a song.
Here is the song Bile Dem Cabbage Down done entirely in the square roll.
The first line uses the "full" sound; the 2nd line uses the frailing sound by dropping the 2nd and 6th notes of each measure.
Try switching the pattern between the two lines and mixing it up in other ways, dropping additional notes or switching for each measure.
There are lots of possibilities.