Clutch fluid a little low

[Stingray1967]

I noticed that my clutch fluid was a little low. See the attached photo. The fluid doesn't look all that bad. Can I just top it off? Or should I completely pull it out and refill? It looks like the way to get it out is to suck it out with some kind of vacuum tool, as opposed to a drain valve.

If the recommendation is to top it off, is there any issue with mixing fluid types? I see both DOT 3 and DOT 4 mentioned. I don't know what is in the car right now.

I should say too, the clutch works just fine, but every once in a while there is a little trouble getting it into 3rd.