Long Range Arduino Water Monitoring
Just in case you consider radio, to get 12km is no easy feat if battery/solar powered. The RF signal power losses in free space are: -

Loss (dB) = 32.45 + 20\$log_{10}\$(f) + 20\$log_{10}\$(d)

Where f is in MHz and d is in kilometres. This equation tells you how many dB of power loss you can expect at a given distance with a given carrier frequency.

The Xbee pro 900 transmits at 900MHz and over 12km the losses amount to 113 dB. On the ground you can reasonably expect this to be worse by probably 20dB taking the link loss to about 133 dB. Some amount of antenna gain may not be very useful if the three sites are at different angles because the antennas become directional. A significantly lower frequency (say) 90MHz (if allowed or available) would reduce the link loss by 20dB.

How much antenna power does the receiver need to operate is the next question. The commonly used formula for estimating this is -154dBm + 10log(data rate). If data rate is 100kbpsec then the receiver needs an antenna power of -104dBm. However, if the data rate is 100 bps then the receiver only needs a power of -134dBm i.e. it's 30dB more sensitive.

Transmit power output needs to be chosen accordingly with link loss (based on carrier f and distance d) and minimum signal needed by receiver (based on data rate).

As for what you are planning to do I would suggest that transmitting blindly once per minute continually is not going to drain battery power over night and so if 50% of the transmissions are "missed" it's not going to be a big deal but you do need to ship any payload transmission with sufficient error detection bits to avoid randomly detecting a stupid status from the remote site.

Just a transmit only radio at the remote site and a receiving only station at the other end should work fine but there's a lot of detail to get right.

See this for more detail.