You already have the money - you don't "have to" do anything. That being said, you have several options:
Repay the $7500
Obviously this is what you're contractually obligated to do and would allow you to sleep at night. When they hired you they of course paid you the bonus with the expectation that you'd stay two years and figured you'd be honest enough to hold up your end of the bargain.
Keep the $7500
You could do this, and risk the chance that the employer would come after you in court. It would cost them a lot of time and effort to get a lawyer, take the case to court, obtain a judgment, and then execute that judgment against you. Honestly $7500 is probably on the low end of what it would be worth to try to go after through the legal system, and the case would probably end up settling for part of that amount (as about 90% of civil cases do).
You'd also of course have burned the bridges at this company if you ever expected a reference from them. That in itself might be enough of a reason to avoid this course of action.
On the plus side, you'd have the $7500 in your pocket.
Stay another 3 months
That way based on the conditions you get to keep the $7500 and you've fulfilled all the requirements - there's no basis for them to come after you and there would be no hard feelings.
Negotiate for it in severance
You can lay out the options for them:
"Here are the possibilities:
- I can stay another 3 months and do nothing because I want to leave anyway to keep the $7500.
- I can keep the money and you can spend a lot of time and effort trying to recover the money from me, which is likely to net you very little and be a huge hassle at best.
- Or, as part of my severance, I'll give back part of the sign-on bonus since I've stayed most of the two years - I'll give you back $3000 and we'll call it even.
Which sounds best to you?"
Negotiate for it from your new company
Tell your current company that by taking the job within the next 3 months you'd have to pay back $7500 of your signing bonus at the last company. Ask if they'd be willing to cover that.
Personally, I'd try negotiating for it in severance first, and then negotiating for it from the new company, if I wasn't able to stay the remaining 3 months.