What can I upgrade to make this laptop faster?

Ok. Yes, an SSD can greatly improve the speed of your system. RAM can too. But everything in moderation.

So here are the doctors orders:

• If you need space but speed, I recommend you remove your optical drive, put you old HDD in there and put an SSD in the HDD's place.
• If all you need is speed, simply replace the HDD with an SSD.
• If you don't want to sacrifice your optical drive, but need plenty of space, you can either get an SSHD (a small amount of SSD combined with a HDD) or just a large but expensive SSD.

Got a that? Good. Now onto the RAM.

• Most people don't use more than 8GB of RAM. If your laptop doesn't already have this, this can also greatly help.
• If your computer has a single 4GB stick, get another 4GB stick to compliment this. If it has any other size less then 8GB replace it with a single 8GB stick.
• If you really need 16GB, get two eight gigabyte sticks.

If you upgrade to an SSD or SSHD, you'll have to re-install Windows. Use this link to make a bootable thumb drive: https://support.microsoft.com/en-us/help...

For all your needs, Dell provides repair manuals. I got the one you need right here:

Inspiron 15 3000 Service Manual

Hope this helps! If you confused about all this, just tell me how much storage you need and what you'll be doing. I will also recommend parts once you've made a decision.