...but I can't see how to change it.
Well certainly not with that attitude! But it's a question of why you want to change. Your cars are paid for, and a lot maintenance costs are going to be pretty proportional to use. It basically boils down to whether your insurance costs+value of space+ depreciation costs are greater than the value you get out of the cars. That's going to be a personal value call in some part. Initially I say: Drop the comprehensive insurance on the classic car. Ditch the truck. Consider ditching the classic car if costs are too high.
The classic car also has comprehensive because I worry about someone stealing it.
Errrrrrrrrrrrt! Stop right there. Reread that. You have comprehensive insurance because you worry about someone stealing the car? Firstly, comprehensive auto insurance isn't going to make it less likely the car gets stolen. Sounds obvious, but the way you're talking makes it sound as though having insurance somehow addresses your worries the car will be stolen.
And if the car does get stolen, your insurance isn't going to cover the real value of the car to you: the sentimental value that it's YOUR first car. Even then it might not cover its true worth: does your insurance agency recognize that it's a well-maintained classic? Or think it's something nearly ready for the junkheap? Their assessment might be way too low, sentimental values aside.
Thirdly, the insurance company knows better than you the chances your car gets stolen and they've priced your plan to make money off of you- it's a losing gamble you've taken. They know exactly what type of car you drive, its security features, its location, statistical rates of theft of the vehicle and vehicles like it. They set the insurance premiums (your wager) to stack the odds in their favor.
Fourthly, if the car did get stolen, it's not as though anything catastrophic would happen to you- you've still got three other perfectly good cars cars. Unless the car is incredibly valuable and you plan to sell it to finance your retirement, why have comprehensive insurance? You're taking a losing bet to get a payout that will never actually cover the sentimental loss.
My guess is that the car will be pretty inexpensive to maintain with only liability insurance. It is taking up space and might be worth something, so you could consider selling it. Admit it, you don't need it, you want it. Nothing wrong with that, but you just need to be aware of what you're giving up to have that luxury.
The truck:
How often do you need to haul a lawn mower or go fishing? You're telling me an SUV can't also hold your husband's fishing gear or a lawn mower? I'd say ditch the truck and use the SUV for that. If you really, really need a truck, just rent one.
I think you can get by with no loss of utility with just the dependable car and the SUV. If you like it, keep the classic, but dump the comprehensive insurance. I'd dump the truck.
If you have a better breakdown of costs for each vehicle, your space, ~miles driven each year in each vehicle or number of days/year each vehicle is used we can help you out better.