The Startup Balance Update 26.03 limits the number of three-point agendas in a legal Startup deck to three:
Corp decks have the following deckbuilding restriction: A Startup Corp deck can only contain a maximum of 3 agenda cards with a printed agenda point value of 3 or greater.
However, the deckbuilder UI doesn't account for this limit, and judges a x6 3-point agenda deck as legal with balance update as the governing rules.