For as long as NFL stats go back, 77 teams have completed at least 70-percent of their passes (minimum of 45 attempts) over the first three games of a season. This would account for approximately one such team per season dating back to the oldest such example I can find, the 1953 Cleveland Browns.
That team completed 56 of 79 passes (70.9-percent) with four touchdowns, one interception and 11 sacks taken.
The quarterback for the Browns that year was Otto Graham, the Patrick Mahomes of the 40s and 50s, if we can safely assume that like Graham, Mahomes will lead the NFL in passing yards five times, completion percentage four times and passer rating five times, including all three of those categories in 1953 — plus also leading the league with 10.