First the Radeon RX VEGA 3 and Radeon VEGA 3 are the same. No difference as far as I am aware of. just different ways of naming the same IGPU.
Vega 3 is the name of Integrated Graphics on a AMD Processor or embedded separately:
AMD Radeon RX Vega 3
The AMD Radeon RX Vega 3 is an integrated GPU for laptops. It is used for the entry level Ryzen 3 APUs, which were launched in early 2018. The GPU is based on the Vega architecture (5th generation GCN) and has 3 CUs (= 192 of the 704 shaders) clocked at up to 1200 MHz (depending on the model). The performance depends on the configured TDP (12-25 W at launch), the clocks, the cooling, and the memory configuration). The GPU should benefit from fast dual-channel DDR4-2400 RAM (contrary to DDR4-2133 single-channel, which is also possible).