It probably is rolling all those dice (as seen by the # of 3d dice rolled), but depending on how a sheet and roll template are designed, which of those dice are actually used could depend on various factors. For example, in PF you roll the attack, IF it's a possible crit, you would need to roll a crit confirm. At the same time, the sheet might also roll the damage as well as possible crit damage. That's a lot of dice that might be included with a single attack roll. The roll template logic will determine if only the attack roll is seen, or if it's a possible crit, the logic might also show you the crit confirm roll and finally depending on those results, show the damage and crit damage rolls. This is all done behind the scenes when you make the attack roll. When 3d dice are enabled, all of those rolls are shown at the time of the attack regardless of the roll template logic that's used to filter what's seen or not seen. Not all sheets or systems make so many rolls at once, but when they do, the 3d dice option falls short.