I tried your rocket
@dezway, and I found it pretty interesting. What's unusual is that your second stage actually acts as a first stage (your rocket isn't fast enough when the first stage is empty, so you still need a powerful engine). That works but this has a drawback: those engines are very heavy (10 tons each). Having an extra Titan for your second stage adds 10 tons of dry mass to your rocket. On the other hand this allows you to alleviate your rocket quite early, so this partly compensates. But in the end I don't think it's worth it.
Personally I would rather use 2 stages, like this:
In flight, here is the situation once my first stage is empty:
Now my ship is fast enough, the less powerful (but more efficient) engine from the second stage is better for the job. In the end, I reach orbit with a lot of fuel left in the second stage:
Now, splitting the second stage into 2 stages would be even better, so that you can jettison some dead weight once you reach orbit
Another possibility closer to your design is that rocket:
I made the first stage tougher, I shortened the second stage to compensate (so that it has the same fuel quantity as yours), and I put 2 Frontier engines. They are heavier than a single Titan engine, but they are much more efficient.
I also put Broadsword engines on the third stage.
@SupremeDorian is right, an efficient engine is best suited for that stage. A powerful engine is useful for the lift-off phase, when you have to fight gravity.
Here is how the flight happens:
First stage empty, I'm still not very fast, but as the second stage has 2 Frontier engines it will be ok:
See? I could reach orbit with a bit of fuel left in the second stage.
But I must admit that it is only slightly better than yours, which is why I said your design was interesting
