My computer is Dell Vostro 3470.
I have connected to 2 monitors: HDMI and VGA.
I want to connect to 3rd monitor. Should I use a 1-to-2 cable slitter? if yes, which port HDMI or VGA is better (or what are the verdicts)?
You cannot add a monitor using a cable splitter on either of these types of output. All that would do is duplicate the output on both of the monitors connected to the splitter.
In order to add additional monitors to this PC, you are going to have to add an additional graphics engine. This can be one that plugs into a PCIe connector (I see Dell offered an NVIDIA GT 710 upgrade card; you will need to ask them what others are supported) or one that plugs into a USB connector. This latter option will be problematic as your USB 3.x ports are on the front of the chassis (the USB 2.0 ports on the back are too slow for a monitor connection). Here is an example of a card you can use to connect a monitor via USB 3.x: https://www.amazon.com/StarTech-com-USB-HDMI-Adapter-External/dp/B00H91BQ7Y.
Hope this helps,