What size are 3mm knitting needles?

First off, there are two "types" of needle sizing. The first is the US method of measuring needles, and the second is the UK/Canada method of measuring. Therefore, there are two answers to your question.

For US knitting needles, there are sizes:

0 (2.0 mm)

1 (2.25 mm)

2 (2.75 mm)

3 (3.25 mm)

4 (3.5 mm)

5 (3.75 mm)

6 (4.0 mm)

7 (4.5 mm)

8 (5.0 mm)

9 (5.5 mm)

10 (6.0 mm)

10 1/2 (6.5 mm)

11 (8.0 mm)

13 (9.0 mm)

15 (10.0 mm)

17 (12.0 mm)

19 (16.0 mm)

35 (19.0 mm)

50 (25.0 mm)

For UK/Canada sizes, there are:

14 (2.0 mm)

13 (2.25 mm)

12 (2.75 mm)

11 (3.0 mm)

10 (3.25 mm)

9 (3.75 mm)

8 (4.0 mm)

7 (4.5 mm)

6 (5.0 mm)

5 (5.5 mm)

4 (6.0 mm)

3 (6.5 mm)

2 (7.0 mm)

1 (7.5 mm)

0 (8.0 mm)

00 (9.0 mm)

000 (10.0 mm)