Interval training 800m x 3

I was planning to do a 800m x 6 interval training with 3 mins rest in between. I set a pace goal of 5:00 pace. I started off at a pretty fast pace of 4:43 despite the 200m uphill slope. And my second interval is at 4:48. Heartrate goes up to 170 at the end of the interval. The 3rd interval slows down to 5:08. And I am already huffing and puffing. Heartrate goes up to 170 just after 400m. I decided to call it off today since I started too fast on the first two. And I will get slower and slower in the next 3. At this pace, maybe I should have 3 mins rest in between instead of 2.