If you're so concerned with fuel consumption, Egan's suggestion of the VW Jetta is a solid choice. It will easily beat both the Civic and Mazda 3 when it comes to efficiency.
The Civic/Mazda 3 does 8 km/L in city driving, while the VW will probably do 9 km/L. Coupled with the cheaper cost of diesel, you will save close to P2,000 a month for fuel.
However, I'm sure you're on the fence with the Jetta just because it's a VW. Being a diesel and being a German car, PMS will definitely be higher than the Civic and moreso the Mazda (free maintenance for 3 yrs), but remember that it has a 15,000 km maintenance interval so you have fewer visits.
Net, maintenance cost shouldn't be a deal breaker and in the long run you will still save on running costs due to the cheaper fuel.
A bigger concern would be long-term parts availability. While personally I don't think premature DPF failure is a big issue in itself, there is still a general difficulty in finding parts for low volume cars. In this aspect, the Jetta is at a disadvantage to the Mazda and Civic. And even between the 3 and Civic, parts are still easier to find for the car that sells more, in this case the Civic.
I have a 13 year old Mazda 3 and parts are a hassle to find compared to my Altis before, not to mention more expensive. Worse, shops in Banawe that you thought were respectable were actually selling fake parts! (I'm looking at you, Mazford!)
I was also considering the Jetta before but went with the Civic simply because it's a simpler car to own for long-term use and because the high tech interior and eyecatching exterior really called to me. If you're willing to take the risk with a VW, you will be rewarded with great efficiency and power, good handling, lots of features, and German engineering all for a very fair price.