How to recharge portal and gain a higher level when i am alone in the city?

If you want to level up, don't recharge your portals at all—wait for them to decay (which should take about ten days) and then rebuild them and link them together to form a field. Repeat this process as necessary.

A better option, and one which would make Ingress an actual game for you in addition to helping you level, would be to find someone to play in the opposing faction who would compete with you for the portals in your town.

Recharging portals awards 10 AP per recharge - whether it is remote recharge that affects all resonators, or using Upgrade menu to target a specific reso. However, allowing portals to decay & then redeploy is still the better choice imho for gaining AP.

• Link all 3 to form a field : This gives you far more AP than just making links between 2 of them. If you don't have enough keys to form the field, better to wait and hope a couple of days' hacking gets the missing keys.
• Allow resonators to decay one-by-one : Getting this right the first time will be tricky, you may have to recharge some resonators etc. But the idea is, as each resonator destructs you replace it - getting the 150 AP for adding it, and 250 AP bonus for filling all 8 slots. If you can set it up, you get 250 x 8 x 3 = 6000 bonus AP for a complete renewal of all resonators in the 3 portals.

If getting another account is an option, then you can find another person to play, making it an actual game as @phenry said ... far better than just you slowly toiling away! Now you also get 65 AP whenever you upgrade someone else's resonator, so with a 2nd person it is possible to alternate upgrades to get some extra AP out of building a portal.

If there are only three portals in the whole city, the best thing to do is to start submitting many more portals. Hundreds of them. I have two portals on the two-block walk to my cafe in the morning, and I think I live in a desolate part of town. I probably have twenty within a half hour walk. And downtown is basically a sea of portals.