How would I use this 12 key keyboard?
There are four rows and three columns which make a matrix of 12 possible connections. Each switch is located at one of the cross-points of the columns and rows: -



If you "read" the value of the voltages on a single column and activate each row with a positive voltage (not together but in turn) you can deduce which of the four switches is pressed in that column.

Read the voltage for each of the three columns and you can deduce which button is pressed of the 12. Two buttons being pressed can confuse things so be aware of that.