As you've already found out, the A minor pentatonic scale - or, if you add the flat 5 = Eb, the A blues scale - fits all three chords. But your solos can indeed be made more interesting if you add more notes. You can use chord tones to spice up your solo and to incorporate the changes into your melodies.
Below are the chord tones that are not already part of the A blues scale and that can be added when the corresponding chord is played:
E7: B, G#
Of course, all chords can be played as ninth chords, which gives you even more options: B for A9, and F# for E9 (the E of D9 is already part of the original A blues scale).
This is a first step which will help you to make your solos more interesting, and which will make your melodies reflect the underlying progression.
One problem I've noticed with my students is that when using chord tones, you always need to know/hear where exactly in the progression you are. E.g., the note C# can be played as a target note over the A7 chord, but not (as a target note) over D7, because then it would clash with the 7th of the chord (the C). (By the way, note that any note is fine as a passing tone). This means that now you need to listen even more carefully and you need to be fully aware of the changes. This is a very good thing anyway, because it will result in more musical solos. However, since exclusively using the blues scale doesn't force you to exactly hear the changes (because all notes fit more or less all the time), many beginners are not very much aware of the changes.
You can use many more extra notes and scales, but I think the above is a very good starting point. Using the major pentatonic scale results in almost the same. Also here you can't use one scale over all three chords, for the same reasons as above. Over A7 you can use the A major pentatonic scale, but it contains the C#, which, as mentioned above, will clash with the note C of the D7 chord. One way to use the major pentatonic scale over a blues is to switch scales for every chord, i.e. use A major pent. over A7, D major pent. over D7, and E major pent. over E7. But this is just one option and not the only possibility.
Only if you feel comfortable with all of the above, you might want to try other things; they will generally sound more jazzy. One option would be the diminished scale (see this answer), or any other scale that can be used over dominant seventh chords.
To hear some weird sounds over the blues, you can check out this instructive example by guitarist Oz Noy. He demonstrates different scales such as the standard minor and major pentatonic scales and the mixolydian scale, but also more oddly sounding ones like the whole tone, the diminished and the altered scale.