JOcean captures the essence of what you need for the gpu upgrade. just to add: you really need to consider improve air flow in the 3881 case when you install a high power gpu.  The case was not designed to handle a big heat load and has a meager exhaust fan to cool cpu only.  Stock psu has no provision to power a custom fan either.  Psu upgrade will give you means to power custom chassis fans. Without additional fan the gpu will be baked to dangerous temperatures in the case and cause component damage as well as performance throttling or system shut down. 3881 motherboard can work with 3060 or up if psu is powerful enough and there is sufficient cooling.  I suppose if you can not DIY custom fans, simply keep the case side panel open and position a desktop fan to blow cold air directly on the gpu will help cool it. 3881 case dimensions 154 mm (6.06 in.). You can install certain SFX 24/4 psu inside the case such as silverstone 500w SFX-L 
Recently want to upgrade my inspiron 3881 with new graphic card (RTX 3060 up) and case power accordingly. Questions: 1.Is it possible for the mother board (graphic card and power)?  2.Is it easy to DIY? Thanks for answering a hardware newb~  
UPGRADE Inspiron 3881 graphic card to RTX 3060 up