Light fixture wiring help
What you have is a switch leg and possibly a outlet or switched outlet depending on how connected, 

the white with the blacks was the switch leg and was correct but it should have Been reidentified as a hot by tape or a colored marker other than white, green or gray.  

Find the cable that comes from the breaker connect that black to the white of the switch if you want the outlet hot all the time connect the don’t know where it goes black to the breaker black.

Now the switch black connects to the 2 blacks for the new light and the black for the existing light. 

Next connect the white from the breaker the 3 lights and the receptacle together.

Last if there are grounds tie them together and if a metal box the box should be pigtailed also. 

This will get you back together. If you want the outlet switched that hot black wire would go with the 3 lamps black but it may feed multiple outlets so hot all the time is normal.

Next time only remove the wires from the old fixture because if it works it is probably correct and you will have to reconnect it that way to make it work in the future.