A bad news
If you did not used separated channels for each ink, you need to do it again.
If you are using layers you have no real pantone information, you just have a RGB file with colors "simmilar" to a pantone.
I never send the Multichannel, but a composition on a PDF file. The way I prepare is puting a grayscale image, but converted into a duotone with only one ink (monotone) and I use the Multiply mode and a opacity at 100% (No aditional transparency) it simulates it quite well.
The diference is that i am using bitmaps with real pantone color on each object, and I can play arround with them as if they were layers, because they are separated objects.
I do this in Corel Draw, but I asume the mode in ilustrator could work well.
Then, when I prepare the pdf file, I export it preserving the internal ink modes.
As a multichanel file is tricky is better to make a simulation of the printed plates. All must overprint, if you want the colors to combine.
I have never sent more than 2 colors overprinting eachother, it could be messy not only because of the adition of inks, but also because the screen angles. I gess 3 will still do fine.
If you have more than 3 pantones... Would it not be safer to have a CMYK print?
Inks in real life reacts a bit different than the simulation, because the simulation renders a perfectly transparent ink, which does not happen in real life, so you need to test the order of the print. I send the lighter one first and after that a clear shadowy area on the top of that.
If the inks does not overlap or overprint, you can send more inks, but beware that they could be possible being printed in a one head machine, which mistreats the paper more. You could have registry mistakes in several passes.
I hope you are talking on Offset commercial print.
A way to change your psd file to real pantone "layers"
On a vector based program.
As I mentioned this process I do in Corel Draw, look for similar steps in Ilustrator or Indesign.
Read carefully the method I use on a vector based program. I am using bitmaps, not only vectors, so this will work with your file.
- Convert a copy of your image to grayscale. Keep the layers separated. There is a chance you will have a problem here.
If your layer is yellow, for example, when you convert it to grayscale you will get a light gray. When you asign a pantone yellow you will have it screened, this is, for example just 20% of the yellow ink, instead a 100%.
If you used masks, change the color of the main layer to black.
Import this multylayer file into your Vector based program (embeded, not linked)
Ungroup the file and re-asign a monotone pantone on each pice.
Use Multiply mode with 0% transparency.
In the case of InDesign there is a chance you need to work your photoshop layers as separated files.
Copy each mask you used on each layer and paste it into a new file as a new channel. You probably need to "invert" each one (negative).
One aditional note
You probably need to adjust the levels again. For example if you have a yellow object when your turn it into grayscale it will be light gray.
When you asign a yellow pantone you will have not a pure yellow but for example a 30% of that yellow, so will be a lot lighter.
If that is the case you need to adjust the levels again so yhat light yellow is pure black, then you can asign the yellow at 100%.
This could cause bandages on gradients, so be carefull. It is better if you start your project right from start.