Finding legal films I actually wanna watch is a hassle and so is converting them to 3gp for my Nokia 5300. If it's not a missing or "unsupported" codec it's the sound gone missing. It feels like the copyrightholders are so hellbent on preventing us from copying their stuff they end up prevent us from watching it at all, especially us Linux-users (you won't need linux to do this though).
But I when accidentally put a unconverted film on my phone it just worked! Turns out that it was the format MP4 on 64kbit that is more or less tha same as "3gp". The file was from Archive.org , an online, public domain library .. sort of. I tried it with another file it works consistently (unlike my spelling).

Step 1: Stuff You'll Need for This.

Hardware :
1pcs Nokia 5300 or another never one that supports videoclips.
1pcs USB-cable or whatever connects your phone to the computer, mine came with the box.
1pcs microSD storagecard, mine came with a 256Mb but I uppgraded to 2Gb.
1pcs Computer with USB-port, I assuming you're using one now :P

Software:
The OS (Linux/MacOS/Win)
Webbrowser

It's possible that there's a different way for you to connect your phone to the computer.

Step 2: Find Flash

1. Go to Archive.org and enter "Flash Gordon" into the searchfield at the top of the page, select "Moving Images" in the scroll next to the search. Press enter or click on GO!

2. Click on the link to Flash Gordon conquers the Universe.

3. To the left are the downloadable files, right click on the 64Kb Mpeg4 and select "Save link as". Save it somewhere you can find it.

Step 3: Connect the Phone.

On my Nokia 5300 I just plug in one end of the USB-cable to the phone, the end other to the computer and select data storage mode on the phones menu. This may vary on your phone.

Next you transfer the file from the computer to the phone. For me this is drag and drop in the filesystem.

If the picture looks unfamiliar remember I'm using Ubuntu but Windows or Mac should be approx the same.

Step 4: Watch the Movie

1. Unplug the phone ..
2. open the menu and select Gallery ..
3. open the flashcard ..
4. open the videoclips (if that's where you put it) ..
5. find Flash Gordon and select Open.

Step 5: Great Success !!

It should be playing the first episode of Flash Gordon Conquers the Universe, complete with bad settings, crummy special effects and all the other really cool stuff.