You don't say if it's a NeoPixel or analog strip.
If it's analog, have a look at
https://learn.adafruit.com/rgb-led-strips.
That guide discusses current requirements, but if you splice together 3 identical strips, the required current will be 3x that of one.
If it's NeoPixel, and a look at the NeoPixel guide:
https://learn.adafruit.com/adafruit-neopixel-uberguide. These are a little more particular to connect together. The Data out of one strip connects to the Data in of the next. Again, 3 strips will need 3 times as much current as 1. The NeoPixel guide goes into things to consider when powering longer strips.
Dave