major scales have a major third interval from the tonic of the scale to the 3rd scale degree
Correct. That is why it's called a major scale.
I would expect the same to be true in minor with a minor third interval but, for example, in the pentatonic minor scale the minor third is from the tonic to the 2nd scale degree.
The numeric designation of interval size (third, fourth, fifth, etc.) is based on the diatonic scale. In the middle ages when these names arose, the pentatonic scale was not on the horizon as a theoretical construct.
That the second degree of the pentatonic minor scale is a minor third above the root makes sense if you think of the pentatonic scale as a diatonic scale from which two scale degrees have been removed. In the context of music theory derived from European tradition, at least, the pentatonic scale is a second-class citizen.
How do you define what a minor scale is?
In general, I suppose, a minor scale would be any scale that includes a minor third above the root (probably excluding those that also include a major third above the root), though I don't think this definition has been formalized, and since I am not familiar with the full catalogue of scale definitions used in jazz theory and derived theories, there could well be exceptions that I'm not aware of.
Certainly in the context of traditional classical theory, the three minor scales are simply the minor diatonic scale, the "natural minor" along with two scales that can be obtained through chromatic alteration of the sixth and seventh degrees, which are the harmonic minor and melodic minor. One also occasionally sees the Dorian and Phrygian modes described as "minor modes" because they have a minor third, so it would not be unreasonable to include them in a broad category of "minor scales," either.
And how do you know where the minor third interval is if it is always from the tonic to the same scale degree?
Remaining in the context of traditional theory, a minor third is simply an interval that spans three letters and comprises a distance of three semitones. If the root of your scale is C then the minor third is E♭.
(This gives another insight into the reason for not calling the second degree of the minor pentatonic scale some kind of "second" -- it would imply that for music using the pentatonic scale we would rename the notes using only five letters, redefining the frequency ratios between the letters. Of course, doing that would be massively confusing. So, for example, one minor pentatonic scale is A-C-D-E-G-A, where the second degree of that scale, C, is a minor third above the root, A.)