Switching 3 different voltages with one input
The way to increase the LM3916's output current is to drive a PNP transistor with it:  

  

That will work as a switch which switches the LEDs on and off. R1 prevents the transistor from conducting through the LM3916's leakage current. You can use a 10k\$\Omega\$ here. 

If you want to control the brightness with your 0-15V you'll need to convert that voltage to a current; LEDs are current controlled devices.
Place the following circuit between the LEDs and ground:  

  

The LED's current will be \$\dfrac{V_{IN}}{R_{SENSE}}\$