Iphone 3gs apps can't download apps?

I've been trying to download apps like tumblr and bbm on my iphone 3gs but it keeps saying that this app is incompatible with this iphone
do you know anything i could to download it/ please help

Simple answer is to jailbreak it. Your iPhone is an older model and therefore can't have the most recent iOS needed. The only way to download these apps would be to jailbreak the phone. Google "how to jailbreak iPhone 3GS" and you'll be on your way, quite a long process though.

It simply means you need a newer device to get that application. You can get this error even if you have the required iOS for the app.