You've got it right , but it's just 3rds not min 3rds in that case ... Wether it's min or maj is determined by each note in the scale.
If you want to use min 3rds throughout , you'll be going off scale , which kinda gives a dark sounding.

Also 5ths are used quite often for harmonizing , you can't go wrong by doing these , cuz they're very flexable due to the fact that they are power chords.