MK3 level 1 is the highest upgrade level available for robots and their equipment: weapons and shields.
The option becomes available when an item reaches MK2 level 12:
Upgrading to MK3 doesn't require Ag or Au and doesn't consume real time: it is instant but an Upgrade Token (or several, depending on the item) is what makes the upgrade to the highest tier possible.
A fully upgraded item is approximately 5% more powerful compared to an MK2 level 12 version and its level is always written in gold: