Dave, we also paid in thirds for income tax whilst in France (and last year for 2013, the year we left permanent residence to move to Malaysia).
The payments requested for each year are based on the previous year and are a first pass estimate. The final total is then calculated based on your actual income as per your tax declaration in May. The last instalment is therefore corrected to make sure your total is correct for the year. In your case you had a zero last instalment so you should not worry that you will get an additional bill (for income tax), in fact you should check to see if you are entitled to a reimbursement. That should have been mentioned in the bill received in July.
The system of estimates basically assumes your income stays the same every year (typical French!). If you stop work (as I did) during a tax year and your annual income goes down then they still request the same amount as the previous year. However you ARE entitled to pay less than they request - based on your own estimates of income tax due. But bear in mind that if you pay less than a third of ACTUAL tax due for each instalment then you will be subject to a fine of 10% of tax due - I don't know if this 10% fine is based on the remainder due or the total that was due for the year. What we did to be sure of not being fined was to pay slightly more than what we calculated for February and May instalments knowing that our last instalment would be between zero and a third of the total.
For our 2014 payments - the first full year abroad - which we are doing this year, we have still been requested the same tax as previous years despite having told the tax authorities several times that we have left France. However we will just pay zero tax.
Don't forget there are also CSG, tax d'habitation and tax fonciere (if you own own home) payments that may be advised separately.
We find it very handy registering for the online service - it is pretty good and all payments due are clearly listed, plus you get alerts by email. Not sure if they do an English version for those that need it but at least you can easily cut and paste to get Google translation.