I also have a 3 yr old, 4 in Oct. We are using this free online program for school right now:
https://allinonehomeschool.com/getting-ready-1/He LOVES it and I feel like it's enough for right now. He's really learning the alphabet songs well with this program and it's sticking in his brain, with all the repetition. He has speech problems, so I don't know if he's behind or where he should be, but I do know he's learning a lot. He also watches the Leapfrog Alphabet Factory DVD a lot.
We will continue with the above program into pre-k, but I will be adding in phonemic awareness activities, phonics and handwriting.
Don't know anything about Horizons. I do like the Sonlight program, but I could never stick to just one program, so I just use a mish mosh of things.