Most recommend leaving the AM3+ platform (as it is already end-of-life/has a dead-end upgrade path) and migrating to current platforms such as the AMD AM4 (Ryzen) platform or the Intel LGA1151 (7th-gen) platform. This depends on your budget for the first batch of upgrades.
Although you can just upgrade the CPU (solely) keeping the AM3+ plaform but, IMO, it would not be worth it as you will be limited in CPU selection and max. possible performance it can offer.
There would also be no point in upgrading to more powerful GPUs if the CPU/platform would hinder the GPUs max. possible performance, so, I would follow common recommendations of changing your platform to give you the opportunity to upgrade to more powerful components in the future.
However, note that you can't really do a piece-by-piece (single item) upgrade based on your parts you listed, if you wish to change platforms. It will definitely a batch (multiple item) upgrade.
Recommended first batch would be changing CPU + MB + RAM (or CPU + MB + RAM + PSU).
Recommended second batch would be changing GPU + PSU (or GPU, if PSU can be included in the first batch of upgrades).
As to which specific models, depends on your budget for such batch of upgrades.