The HK4 had 4 calibers (3 replacement barrels), but no extra slide was needed. It is the gun I have most often seen as a set with all the calibers in it. The HK P7K3 had 3 calibers, but only 1 extra slide (for .22LR).
The Desert Eagle can come in a kit with 2 extra barrels and an extra bolt to give you 3 calibers.