Both are classic Windows client/server products.    No Saas.  The only thing they have in common is the word "sage" and number "300".  One is gasoline, one is diesel.

Sage 300 CRE and Sage 300 ERP are different products, there is no common ground to share technically.  Is your CRE running on the Pervasive DB engine?  The only way I can think of to share an integration would be to use the ODBC API and classes to push and pull data between both using Microsoft SQL and Pervasive SQL ODBC drivers.  Or if its a one-way out integration you could replicate the Pervasive data out to Microsoft SQL and interact with it from there.  The huge downside of this method is you lose access to objects and functions within Sage 300 ERP that make integrations much easier, and writing data back is very difficult - by bypassing the business logic layer and writing direct to database you have to be very careful to not corrupt the databases.

Ideally you want to write two integrations, one unique to Sage 300 CRE and one unique to Sage 300 ERP.  I of course in this forum can only provide advice on the Sage 300 side.

For Sage 300 if you're more comfortable with modern API REST integrations using JSON packets then as long as your client deployed the Sage 300 web screens there is a full API available with documentation located https://<server>/Sage300WebApi

Or you can write an integration using the Sage 300 COM objects which requires a copy of the Workstation Components to be installed on the development machine.  Google turns up some articles but someone may have a more official guide to help you get started, examples:
https://www.netatwork.com/newsletter/sage-300-newsletter-april-2016/the-basics-of-connecting-to-sage-300-using-the-api/
https://communityhub.sage.com/us/sage300/b/sage_300_erp_r_and_d/posts/an-introduction-to-the-sage-300-erp-net-api

Good luck!