This movie is not about the three (or four) musketeers. It is a funny parody of all those movies, and it centres around the four servants of the musketeers. As a kid, I enjoyed it immensely, and it was the only musketeer movie I liked, simply because it is such naive and harmless fun.
The main plot: The three (or four) musketeers are useless. They are lazy and drunk and barely ever appear in the film. It is their four servants who do all the work and all the rescuing. And they faithfully tell the stories to Alexandre Dumas, the pub landlord who likes to write things down. The rest sticks to the main premise of the novel (a French queen in distress, an English Baron dating her, etc.) but always with a wink in the eye. This is a slapstick movie, a parody and a funny one, although not really a clever one. The finale is big fun, involving lots of people sneaking up on each other and kicking each other in the butt, so you can see that this is not clever or subtle humour at all. Think of the average Bud Spencer & Terence Hill movie and you're quite close to this. I'm not sure whether it has as much appeal to adults as it has for kids...