Selecting notes

To create a triplet, select one note or rest with the total duration of the triplet, then press ctrl+3. For example, to create 3 8th notes in a triplet press 5 ctrl+3. You can then enter the three notes of the triplet or any other appropriate rhythm like a quarter note followed by and 8th or 6 16th notes etc.

As suggested above, you don't create triplet by first entering the three notes then pressing Ctrl+3 - you do it by first pressing Ctrl+3 then entering the three notes.

But for the record, to select a range of notes, click the first, shift+click the last. Or click the first, then use Shift+right to extend the selection a note at a time. Same shortcuts as in most text editors and other programs.