I have a file with three weight masters: Thin Italic, Medium Italic and Black Italic. One axis, three masters – three layers. From that, do you suggest to me to add other weights, for instance: Light italic, Bold italic and so on to the same file. If so, I will have only one file with ten weight which I want to create.


No, there is no need in other masters. You can define instances instead.

Masters are real font designs with nodes, contours, kerning pairs etc. and you already have 3 of them on one Weight axis. All other intermediate designs (between wt=100 and wt=900) can be generated automatically at any time using interpolation algorithms. You can define font instances that are declarations of future font styles, just coordinates on the axis for future interpolation and export.

Your Thin is at coordinate 100, Black is at 900 on the axis. On the Font Info > Instances page, you can declare what other styles with what coordinates you will want to generate in future. For example, Bold at 700, ExtraBold at 800 etc.

Here is the quick way:

1. Open FontInfo > Axes, click on menu button at the right and select "From Axis Defaults":
This will populate the list of default instances in your Weight axis.

2. Now open FontInfo > Instances and click on "From Axes". This will populate the list of instances in your font with instances from the axes page:

You can now click on every instance in the list and see its properties. There is nothing to change at this point though.
Instances are not "real" designs. They are just the recipe for interpolation and font creation. "Real" designs are your masters (Thin, Medium and Black).

3. Finally, switch to the Masters page and make the Medium master the default master using the blue dot at the left of its name:

4. Click OK to save changes and close the dialog. Save the font.

Now to generate all your font styles (real and declared as instances), open the Export Font As dialog, click on Instances tab at the top and select what particular fonts you want to generate (blue mark at the left):