Here's some friendly advice from me

1st song:
-Nice melody, but it needs some variation because it gets very repetitive over time.
-There are a bit too many high frequencies. Some EQing on the hats could fix that.
-The strings sound very mechanical and not realistic. I suggest lowering the attack and having them play more chords to get that realistic and non-mechanical feel. I personally use Kontakt's strings which are very nice, and I recommend them.
-Nice kick and snare!
-You play the same melody throughout the whole song, which like I said before get repetitive. I think it would be helpful if you cut out the melody at some times and maybe added a bridge. For a about 2 minute long song, it's not bad, but some variation could do wonders! :3
2nd song is gone, so I can't give advice on it.
3rd song:
-The hat seems to be so loud that it detracts from the rest of the song. Decreasing it's volume would help.
-I like the beat!
-Nice bass!
-I think having a lead would be nice in this song, because the bass is great, but gets boring after a while.
-Seems like it needs some other sounds in the background to spice it up a bit.
-It's definitely not as repetitive as the first song, but adding some variation to this one as well could do wonders.
Some bonus tips:
-Panning everything except for the drums can have a really nice effect. It makes there seem like there's more space and the song won't feel so static. Just don't pan to much, or else the track could be hard to listen to!
-When mastering, EQ each instrument. It's important that instruments don't collide with each other in the frequencies and that you bring out the right parts of each instrument.
-I've learned that taking out all frequencies below 60hz also helps.
Wow I typed a lot. Hope you like the feedback!