I think it really sucks that you cannot request help.
Defensive pacts are good but they come late so DoFs can buy you time as they can't attack during that time. But, the main way is to joint war someone first. The reason why you are being ganged up on, is they're doing a joint war against you. If you actually can't fight a war, then make sure they're a bit further off.
For example, I had a game where I didn't like France or Brazil. I noted that Kongo hated Brazil since the later took a city from them. So I decided to joint war with Brazil against France, and a bit messing around later, made peace with France. Then I joint war with Kongo against Brazil who is more than willing to do this, and target the city Brazil took from Kongo. This earns me a big diplomatic bonus with Kongo and he stays an ally for the rest of the game as well while preventing France and Brazil, from say ganging up on me, since they all hate each other now.
The weirdest thing was that Pedro didn't seem all that upset, and even granted a liberation bonus (???) for taking that city
You can also see that I wouldn't be on good terms with Kongo normally.
Now obviously not every situation is going to be ideal, so always be ready to defend your improvements and districts and try to put your cities so they'll have to funnel through. I like settling crap cities near people and build an encampment which often buys enough time for me to protect my valuable stuff. Just promise to not settle as close (you can't anyways)
