It is very, very common for handwriting skills to be behind reading skills. They are on different developmental trajectories. If he's not complaining, you can keep doing whatever you're doing. If his handwriting ends up that bad, despite your attention, it's going to be dysgraphia and there's only so much one can do about that. At this point, though, neat handwriting is unrealistic. He's only 3.
The best thing you can do really at this age is motor skill work. Fine motor skills are reliant on core muscles, so all the gross motor skills like running about like a feral at the playground and climbing the clothesline are important. This is all the behind the scenes, foundational work for handwriting and a lot more fun for both of you than correcting handwriting.
As for grip, that should develop on it's own but in some places kids are taught to write using fountain pens, so the pen can correct their grip instead of the adult nagging. There are pens made for kids, but I forget the brand we used.