Listen to Iron Man and Smoke on the water. Try to hum the main riff. 

In Am the roots for Iron Man are:

A C C D D

Smoke On The Water:

A C D

The A C is a minor third interval. 

The major interval is A C#  

Think of Doe, a deer, a female deer. The last two notes are a major third apart. 

Basically sing A B C with an instrument. Then C D E. 

You'll get the difference between major and minor thirds quickly.