What I teach my debaters is that the "three voting issues" approach is actually a really bad idea. If it's what your judging pool expects, and they respond badly if you deviate from it, then take that into account, but here's a different way to approach it.
A drill we do over and over and over and over again, until my debaters are thoroughly sick of it, is "What issue" -- issue, singular, not plural -- "does this round turn on?" We stop before the last rebuttal, and I ask them, and we talk about it. What single issue will decide the debate? Once that's identified, there's a simple if-then process. "Are you ahead on that issue?" If so, proceed to the next paragraph. If not, skip to the one below it.
If you're ahead on the issue that is the turning point in the debate, then give the most vivid, concrete explanation possible of why you're ahead. Fairly summarize your opponent's position on it -- fairly, no strawfigures -- and then explain in a summative way, eloquently, with concrete, well-impacted arguments, why your opponent's position is inferior to yours.
If you're behind on the issue that is the turning point in the debate, then what issue represents your best chance of winning? Let's say you're affirmative, and the negative is absolutely piledriving you on a solvency turn. Is there another issue in the debate that gives you a glimmer of hope? Is there a part of your case that's independent? Did you turn a disadvantage that at first glance doesn't seem to outweigh the solvency turn? The wrong move is to just reexplain your position on the issue that you're losing. The right move is to take the single issue which best combines two elements -- 1). your position on it is far more cogent than your opponent's, and 2). it has the most potential to outweigh other issues -- and start on that issue, speaking expansively and vividly about why you win it and why it matters. Then, put a generous investment of time into the best possible damage control on the other issue that you're losing, and hope for the best.
Whichever of the above two approaches you take, they account for most of your rebuttal, but not 100% of it. With the time remaining, you tie up loose ends and put entire issue areas in perspective. If there's been a back and forth, yes-no debate on your harms, or your plan's degree of detail, then you sum up your position in a sentence or two, and explain quickly why it's not as important an issue as the issue you selected for the above approach.
Last thing, to answer your precise claim about being top heavy and spending too much time on the first issue. I have a debater right now who has the exact same problem: every single time she gives a speech, she's slow to get going, so the first issue gets over-discussed and everything that follows gets just a cursory few words. There's a two-part training regimen for that. First, we do a drill right after a speech where I have her explain her argument on an important issue while I run the stopwatch. She explains until she feels like she's done, and I stop the stopwatch and tell her the time. Let's say it took a minute and a half. I say "Okay, same explanation, but this time you only get forty-five seconds." I call it off by the fifteens: "Fifteen ... thirty ... stop." Then I make her explain it again, but I give her only twenty seconds. Then I make her explain her position in a single sentence. What I'm trying to do is wake up the brain-muscles for boiling down an explanation, making it tight and concise. I'm putting her through the condensation drill over and over until she can do it from sheer familiarity.
The other thing is, I remind her before every single speech in a practice debate, "Start tight." Her challenge is to get that first issue covered in full-out condensation mode. Her natural tendency to start slow collides with her developing skill in tightening her explanation, and it winds up being about the right amount of development and elaboration. The progress is coming slowly, but it's noticeable.
Give that a try and see if it helps.