Robert A

You’re right. It is easier when there’s a load on my legs.

rubiksoval

With heart rate, you're going to have to approach these intervals differently, especially at 3 mins duration.

For the first interval, if it's appropriately paced, you probably won't get to 158. Maybe by the end of the second or third interval. By the fourth interval you may be getting into that range around 2 minutes in and subsequent intervals may have you getting into the range after 90 or so seconds depending on your fitness.

If you're hitting 90% earlier than that, you're very, very likely going out way too hard and then dropping off power significantly at the end.

Especially if you're only doing three, which is not enough to really start doing much.

If I were you, I'd shoot for five intervals next time with 2.5 to 3 minute recoveries (no more recovery time than that, for sure) and really focus on a linear heart rate progression where you're trying to maintain a steady effort and have the hr ticking up 1-2 bpms at a time. And again, if you're hitting that 90% range anywhere but the very, very end on the first interval, you likely went too hard.

Heart rate is a slow responder to short, high burst, and three minutes is on the short side for higher intensity work such as that. Plus, the fitter you are, the slower the response may be in training (adrenaline can peg it right away, though).

Carbonfiberboy 

As rubiksoval says. I've done intervals for years with only a HRM. What you do is do intervals on hills with a steady gradient. Then just hold the cadence = hold the power, and basically ignore your HR. Note your speed, too. After a few tries, you'll see what your HR does under that condition. If you blow up, go a little slower. I try to use a hill that's longer than I need for the interval time. When I'm fit and doing a set properly, the first rep's tough, the second a little easier, and then they get tougher again. That's holding speed and cadence for the duration, each rep the same. When I can't get as far up the hill as I did on the second rep, I quit. Hopefully, that's on the last prescribed rep. If I wasn't able to complete the set, I'll know to go easier next time and vice versa. So that's the free power meter approach. Your HRM will still give you valuable information, but you should ignore 90%. It is what it is, or better, it will be what it will be. For every interval duration, appropriate HR will be different. Learn what those are and also, watch them change!

There is some disagreement about making the last rep the one where you couldn't make it all the way. Some feel that it's better training to force a couple more reps even though they aren't perfect, sorta like weight training. That may or may not be individual. In any case, hold the same speed/cadence for all of them. Don't slow down.

I have some disagreement with those above who say back it off and let them go. I tell my newbies to hold the wheel until the blood starts from their eye sockets. You want the intensity. When I was starting out with my local fast group, I rode the last 20 miles or so solo many a time. Just make sure you know the route ahead of time or have a cue sheet so you don't get lost.

Another thing you can do is to simply hold your max intensity as you go over the top. They'll usually back way off after they go over. You don't. Often you can get them back and recover as last wheel. Trying to hold intensity for long periods is really good training. Obviously all that advice is useless on climbs of more than a few hundred feet where you really do have to back it off and just do what you can do, trying to hold the same pace all the way up. That's where doing hard intervals on your own really helps, because you'll be getting more intensity there than you can manage on long climbs.

guachi

I was doing a race for the Zwift Training Academy just for something to do that was different than my usual free ride on whatever course seemed interesting. The races were fairly short, as it turned out, 14 minutes and 18 minutes. While neither was climbing (though there were short hills) I did push myself far harder than I otherwise would have just because I was trying to hang with the pack I was in.

In other words, I endorse using others as an excuse/reason to go as hard as you can for as long as you can.