I think both calculators are right because they are using different assumptions. If you made $75,000 for the entire year, then your W-4 should read single with 1 allowance, for your particular circumstance. But if you were not working for the first 5 months of the year, and then take a job at 75K, your actual income for this year will be around $40,000. The standard deduction and personal exemption are amortized over a shorter period and have a larger effect, so it could indeed be the case that Single/3 brings you close to break even.
If you keep it at single/3, you will get more take-home pay now and a smaller refund, but you will have to change it to single/1 on January 1, 2016 so you aren't under-withheld next year, and that will cut your take home next year. If you change it to single/1 now, it will give you more time to get used to the lower take-home pay, but will give you a larger refund for this year.
