4GB of memory is not enough for most applications nowadays. I would at least double it, but replace both memory modules in pairs to avoid an unstable system. You can add a M.2 NVMe SSD to your system. Moving your Windows installation onto it (or reinstalling Windows from scratch) will provide much faster read and write speeds compared to a 2.5" SATA SSD drive. You can keep the original hard drive installed and reuse it for saving personal files that would consume a large amount of space on the M.2 NVMe SSD. No, these upgrades won't be overkill for a i3 processor. You will see a big performance increase after performing these upgrades. The slow access speeds of the hard drive is what mainly bottlenecks the performance your computer, but a limited amount of RAM can also bog down performance as well. See the online Inspiron 3584 Setup and Specifications PDF for memory details.
I bought a Inspiron 3584 laptop with i3 7th gen processor; 4GB RAM, 1TB HDD, 2GB dedicated Radeon Graphics with Windows 10. Laptop is too slow for even using multiple MS Office packages at a same time. So I'm thinking upgrading my laptop rather than buying a new one as I bought this 2 months back. I am thinking of adding another 8GB RAM (12GB after upgrade) and a 250GB SSD along with the existing HDD.  Will this be a worthy upgrade or will it be an overkill for a i3 processor? Any other upgrade ideas will be helpful too. Thanks in advance. 
Inspiron 3584, ram, SSD upgrade?