Scale parameter

as far as i know, there's no 3D parameter that looks like what you describe.
many effects that need to define a point in 3D use a point param for the x and y and an additional slider for z.
I totally agree with you that it's a pain in the behind.
having to tie the x and y separately for the z and defining which channel goes where... it's a repeating head ache.

I can think of three solutions for you:
1. create a custom parameter.
you draw the param yourself so you can make it whatever you want.
2. instead of having a parameter that describes a 3D attribute, use a layer selector and read that layer's scale (or position).
you can even have a button on your effect that creates a null object and selects it automatically.
3. use a color param, it can hold 3 or 4 parameters with one keyframe.
in fact, you can use a color param with a custom interface and just draw the data there. the problem with a color param is that it's limited in range. it can't go negative or past 100%.

in my opinion a custom param is the way to go.

Thank you for the answer.
I can of course add three float sliders, and that s what I m doing right know.
But, if you want to link this parameter to a scale attribute of a layer, you will have to edit the expression, and you can not link directly with the UI. And you have three lines in your UI instead of one.
you will have to write something like:
temp1 = effect("MyEffect")("Scale 1");
temp2 = effect("MyEffect")("Scale 2");
temp3 = effect("MyEffect")("Scale 3");
[temo1, temp2, temp3]
in the expression field of your scale attribute.

I would like to be able to define a parameter the same way layer scale parameter is defined, so that user can use the "Expression pick whip" button...

How about using 3 float sliders?
PF_ADD_FLOAT_SLIDER
Then specify the range from 0-1 or 0-100.