3 months with 3G in countryside

Hello
Budget traveler without japanese skills to hike in Japan for around 3 months is looking for data plan:
- speed is no issue (whats app, occasional info from web, no streaming)
- 3G acces in wide countryside is convinient!
- not too expensive, cheap is better

Is there any 3G coverage map based on frequencies?
(still not sure if buy Htc One M8 or Xiaomi Redmi Note 3 Pro).

Parrently:
Data sim of b-mobile uses Docomo network, which is wide coverage network, right??
Will be easier to get Softbank data sim on airport?
Econnect is recommended here, but no 3 months rental.
Another solution?

Can you explain how to get rental sim physically into hand, once arrived to Japan?
Thank you
(Probably flying to Osaka and spending day or 2 in there).

Docomo has the widest coverage area, especially if your phone supports Band 6 (800MHz).

All the other questions are answered on providers' websites.