Sell your 3G cellphone now?

[Geek-9pm]

Not news. 
You already know the 3G thing is going done. It may come sooner than you think.
Here is a link that claims to have the latest dates:

AT&T to shut down 3G network in 2022; Verizon at end of 2019

The demise of 3G in the U.S. has been all but certain after cellphone carriers spent billions of dollars over the past decade to blanket the country with 4G service. That standard, also known as long-term evolution, or LTE, allows users to download data 10 times as fast as its predecessor and has paved the way for many smartphone apps that require ample mobile bandwidth.

There is no effective way  to make a 3G phone into a 4G. You are stuck with your old 3G only thing. So maybe you should sell it.
Here are some ideas:

What to do with old cell phones?

You can not just toss t into the trash. 

What will you do? 

[Salmon Trout]

Most European countries are keeping 2G (GSM) so a 3G phone would just "fall back". I don't know about the USA or elsewhere. A 3G phone is effectively a candidate for the trash anyhow. A bigger concern is for operators of systems using 3G for connecting "internet of things" devices, that is, things like household electricity and gas meters, fire or burglar alarms, but they have had years to get ready.

[Geek-9pm]

From what I see, there is not enough spectrum space for many kinds of cell phone networks. I think it means that it is not very effective for an expensive cell tower to handle older network types. Apparently the older equipment uses power and spectrum space tah could be used for the 4G LTE system

The slurp from Verizon might help a little.
What Is 4G LTE?

LTE stands for Long Term Evolution. It’s a term used for the particular type of 4G that delivers the fastest mobile Internet experience. You’ll usually see it called 4G LTE.

Which implies that 3G does not have a long life. 

[Salmon Trout]

Yes, LTE is what all the 3G users are moving to.

[Lenard]

Hi guys over here in England we now have 5G so 3G is kind of ancient to us now but, we still get 3G when reception is a bit poor specially in the rural England. When it comes to 2G boy can not remember the last time i heard of it.