SMD LED is generally classified by its size( eg. 5050= 50mm x 50mm, same for  1210 ), 5050 is the only type can be mounted into 3 LED Chips, please see following   illustration:

blankNeither 3528 nor 3020 has enough space for 2 or above chips, to judge the qty of  LED in one bulb, necessary to calculate the numbers of LED chips but not LEDs ( eg.a bulb with 12 x 3020 SMD= 12 LED chips with 12 emitting points, a bulb with 12 x 5050 3 CHIPS SMD= 36 LED chips with 36 emitting points) When people say that   there are XX pcs SMD LEDs in one bulb, we have to notice what kind of SMD it is,   because it affects how many LED chips.

3020=3mmx 2mm   Reflector Size

3528=3.5mmx2.8mm Reflector size a.k.a 1210(.12inch x .10 inch) in the market

*Reflector: the carrier to carry the LED chip, the white part as you can see around the LED in the picture. These 2 SMD LEDs usually has no difference because there is only.