Short answer is no, there’s no activation period and you can enter any time between the date the visa is issued and the day before it expires in 5 years.

The 3 month/90 day thing comes from people conflating embassies not wanting to process applications with an intended arrival date of more than 3 months out and single entry visas such as Non-Immigrant O, B and ED or a SETV which are valid for 90 days.