If you're willing to mention the area you live in, or even the specific grammar school, it will help people to advise. They are not all equal.
Regardless, I'd agree with others that have said that Y3 is too soon to do any specific preparation. However, there are things you can be doing at home in your day-to-day life that will help your son to thrive at school, which stand him in good stead if you do decide that he will take the 11 plus.
For me, the main thing is reading. Read with him every day (or very nearly every day) - take it in turns to read. When you're reading, model good reading - expressive, showing an understanding of the text, and perhaps using different voices/accents for the different characters in any dialogue. Ham it up, and encourage him to do the same. Laugh at the funny bits.
When he is reading, make sure it is 100% accurate. Go back over any guessed words. If he is rushing through and might miss a word or add a word as he is guessing the meaning, ask him to repeat. It is so important that reading is accurate. You can even add some deliberate mistakes when you're reading, so he can correct you!
Also, develop his vocabulary. If there is any new word (or even a word he thinks he knows) ask him to explain exactly what it means. Then tell him what you think it means. Then look it up in the dictionary (and old-fashioned paper one, if possible) and see if he/you were right. Then suggest alternative words/phrases the author might have used, then look the word up in a thesaurus and see if there are more alternatives available. Then try to decide why the author chose this word rather than the other alternatives.
For comprehension and inference, ask him what is going on, how a passage made him feel, what might happen next. Enjoy the 'guessing' games. Discuss whether he is enjoying a book, and why. Spot differences between writing styles between authors.
At this age, the 'quality' of the books probably doesn't matter too much. The school book might be suitable, or whatever he is into. But do try to vary it a bit. Perhaps some non-fiction and poetry sometimes, and a range of authors.
This might all sound like a lot of work, but if you can be enthusiastic about it, it can be really fun - and it is so much more valuable than working through flashcards and endless comprehension pieces in a panic in Y5 to prepare for the 11 plus. He will find this part of the 11 plus easy if he has done 'intelligent' reading throughout primary.
Others can advise better than I can about maths, but the same principle stands - if you can include some maths in everyday life, it will really help.