Each NeoMatrix board has a data-in pad on one corner and a data-out pad on the opposite corner. The matrices accept a 5v signal via a single wire protocol (you don't need to know much about this to get started - the NeoPixel NeoMatrix code libraries do the heavy lifting here). When tiling, data is passed between matrices from data-out to data-in. One thing to note is the resistor placed before the first matrix's data-in - this supposedly prevents voltage spikes from burning out the first pixel.
1) Soldered a resistor directly to the data input pad on one of the matrices, soldered a wire onto that resistor. This is the wire that connects to whatever pin is outputting data on from you Arduino.
2) Wired the data out pad of the first matrix to the data in pad of the second matrix.
3) Ditto from the second matrix to the third
2) Soldered a wire to the ground pad adjacent to the data in pad. This wire will go to the ground pin on the Arduino.
3) Wired all grounds pads together.