There's almost too many to choose from, but there are a few that instantly came to mind when I saw this thread. Not saying that these are the Top 3 goals over the past 30 years for United, but these stand out to me for a mix of reasons like importance, skill and iconic commentary;
1. Macheda vs Aston Villa
2. Rooney volley vs Newcastle
3. Rooney overhead vs City
Honourable mentions:
Ronaldo vs Porto, Scholes vs Villa, Van Persie vs Villa, Berbatov overhead vs Liverpool, Beckham vs Wimbledon, Giggs vs Arsenal (FA Semi's)
Probably showing my age here, but can't remember much of our stuff from the 90's that wasn't in highlight reels or Premier League Years...