I would change the beginning to "I have made a decision to". I would change the ending to "if I try to do all three at once it just won't work."
I've come to the conclusion to focus on one thing at a time, because running a business, going to adult school, and taking care of the kids is just too much, and if I do all three at once it just won't/wouldn't work.
Are both okay here?
It is better but you dropped "all" from "all three". I would also consider breaking the sentence into two. Try a period after "much" and then start the next sentence with "If I...".
Why is it won't and not wouldn't? Maybe some people can handle all three tasks at once, so wouldn't that make it a possibility?
Whereas if he had wrote:
If I tried to do all three at once it just wouldn't work.
He hasn't tried it yet, and isn't even thinking about trying because he thinks that he won't be able to handle it.
Is this so?