To display quarter note triplets, you must first set the display quantization to a triplet value. You can set the display quantization from the Display Bar, Display Track or Display Score dialogs, available under the Display menu.
By default, quarter note triplets display the first triplet as a quarter note, the second triplet as an eighth note tied over the beat to another eighth note and the third triplet as a quarter note.
To display as three quarter notes, select the arrow tool in the toolbar, and select the first of the two tied eighth notes by double clicking on it. From the drop-down edit menu that appears choose Ties and then No.
Alternatively, to display as three quarter notes, set the beat for the measure containing the quarter note triplet to a half note. This will ensure that notes are placed in the measure in half-note groupings. To do this, put the cursor in the bar containing the quarter note triplet, choose Display Bar from the Display menu and then change the Beat option to Half. To change the grouping back to quarter notes (or whatever value you were using) in the next measure, you must move the cursor to the next bar and from the Display Bar dialog change the Beat option to Quarter (or the value you were previously using).