Will a dll targeted for .Net 4.x work with Core 3.0?

If the Portability Analyzer reports no Core 3.0 portability issues for a dll compiled for 4.x, can that compiled 4.x dll be used as is with the 3.0 Core framework app?

This question is really about 3rd party components and the amount of pain/need in finding 3.0 Core compatible components.