Thanks to everyone who posted.
You've given me stuff to try.
Tomorrow I will start again!!!!!
Echo

The bass and drums VU thing is a good thing to know.

Also, look up the pink noise trick (I think on recordingrevolution.com). Add a track of pink noise at around -20. Solo this with one track at a time, and bring the track's trim up so you can just hear it pop up over the pink noise. After each track is done, mute the pink noise and play all tracks.

Try balancing your mix in mono through cheap speakers, no plugins, balancing on the trim control not the faders. Pop it into stereo on good speakers and you'll probably be most of the way there.

And not to forget about it -
panning could also  be a reason for umbalanced mixes -
think the experienced user  know how to .
Have you thought about ?

Echo
Hello
I'm sure this has happened to you.
You've got three tracks ie bass/vocal/piano.
You balance them and the bass sounds too loud.
So you lower it and, oh dear oh dear, the other two get louder!
So you lower them a bit and , oh dear oh dear, the bass is too loud again.
Before you know it, you've pulled everything to the bottom of the scales
and you start all over again. I seem to have had this problem all of my recording life.
Are there any definitive answers to this problem?
Sorry if you've heard this all before!
Echo

This is the point at which you stop.
If you play with the Bass level and slowly bring it back up, you will find the happy medium for all 3.
 
I believe someone mentioned this earlier but its crucial, sometimes we don't need to move a fader to make things lower in the mix. Use EQ.
By cutting the bass around 80HZ, you can make it fit in the mix a lot better than if you just start fader moving.
 
The same can be said for your vocal. By adding some 7K, you will brighten the sound of the vocal to cut right threw the mix. However one needs to be careful with 7K because it will enhance sibilance. (the Ssss, the Shhhh, the T's).
its even more enhanced when you use compression.
These are just some things to think about.

I'm with Chuckebaby with EQ suggestion. Pianos lower part interferes usually heavily
with bass, and if the kick adds to that, it's practically impossible to solve it using level faders.
The arrangement plays a big role of course - what is played on the left side of the piano keys.
I've sometimes used EQ and volume automation so, that when the bass plays something I like to be heard,
I EQ the low piano practically out, and vice versa. Or EQ one of them with a narrowish band pass.
 
 

... Band reject?

Sanderxpander
... Band reject?

A Band Reject filter may be too much.
 
I'd be happy just to use the Parametric in the Quad Curve with a fairly wide (low value) Q
 
Another useful trick is to use Voxengo's Span (free) which can be easily set up to show the curves of 2 different audio tracks simultaneously making it easy to spot overlapping frequencies. You'll know then what and where to cut (or more rarely, boost)

A parametric EQ used for cutting is basically a band reject filter with variable strength. I was just replying to the post before that suggested a narrow band pass. I figure he meant a narrow band reject (or high Q EQ cut).