Speed is relative - I think 5.8 is pretty fast on the treadmill - esp. for a beginner runner. You may want to slow that down and see what happens to your hr. Also, you aren't running at 100% of your max hir - it is most likey higher than you think. When I first started wearing my hr monitor I got very freaked out by the hr's in the 180's that I saw. I asked my doc about it and she asked lots of questions about how I was feeling - any chest pain? - no, any dizziness? - no, feel nauseaus? - no. Anyway, you get the picture. I did find though that if I kept my hr up that high for my long runs (over 4-5 miles), I would be exhausted for the rest of the day. Your best bet would be to see your doctor and get their advice. A little trick to find out your max hr is to find a hill (or set the treadmill on 4-5 incline after you have warmed up) and run it for 1-2 minutes. You should get close to your max hr doing this - but you don't want to do it without a clearance from your doc first! Then, try to keep your hr at 60-70% of your max during your runs. You will know when you hit your max - you will be struggling to catch your breath and you won't be able to maintain it for more than a few minutes. Doesn't sound like you are there yet in your 3 min. runs. Also, as you get more fit, your hr should settle down. Mine still gets up there when I do speedwork, but it stays in the 120's when I do a brisk walk and the 150's on my "easy" runs.

Hope this helps.