If there's 5v going in, but no 3v3 coming out, then it's probably broken ... unless there's a short-circuit on its output pulling it down.
To rule that out, you could measure the impedance across its output, ... but you need to know what impedance a known good Pi has, which is where a replacement Pi comes in handy.
Even then, if you've managed to damage the SoC, by (perhaps) putting 12v onto a GPIO - maybe the SoC is pulling the 3v3 rail down, giving you the symptoms you describe.
Replacing any or all of the 3v3 supply wouldn't restore your Pi
Then again, what if it's not something on the 3v3 rail that's killed it - perhaps it's the 1v8 rail that's got the problem.
Again, replacing bits of the supply won't fix it.
One of the nice things about working in the TV industry was the ready supply of spare parts.
If a fault wasn't obviously down to one particular bit, swapping things out until it all started working was the quickest way to fix it (and speed was of the essence when the Derby runners were already on their way!)
On occasion, though, we didn't have that option.
In one situation, some Sony equipment used a Sony-designed chip that was prone to failure, so we were required to remove the offending item and return it to them for free-of-charge replacement.
Being multi-legged (somewhat similar to a SoC) that was quite difficult, and many an expletive was spoken, just to save the company fifty quid