Typically AWS recommends mimimum leg size of fillet welds to be 3/16" with plate up to 1/2", and the min leg size need not exceed thickness of the thinner plate.  The parent material tears out, negating any gains for larger welds.

The max effective weld size of 1/4" and less should be equal to plate thichness

Ref(AWS Bllg Art 212(a)2, AWS Bridge Par 217 (c), AISC 1.17.5), Design of Welded Structures, Blodgett, ch7

Rules may vary from locale to locale.  Check your appropriate codes.

Where I live, a 3/16" fillet is minimum when plate thickness is less than 1/2".  

When plate thickness is btwn 1/2 and 3/4, 1/2" fillet is minimum.  When plate thickness is over 3/4, 5/16 is minimum.  

The above welds must be done in one pass.

The above rules can be diregarded if you have established proper welding procedures with tests.

I've done a lot of structural work and have never specified a 3mm fillet weld.  It's just too small.  5 mm is the smallest I've ever used.  If the operator misses by only 1/16" then you have lost 50% of your strength.  If it's really important to keep weight down or if you are doing some mass-production of light-weight work, then it may be economical for you to develop welding standards for a smaller weld.

In general, it is poor practice to call for welds which are larger than the thickness of the base metal.  Check your stresses in the base metal and then in the weld metal.  You'll probably find you don't need an extra large fillet weld.  

Oops.

Above should have read when plate is btwn 1/2 and 3/4, 1/4" fillet is minimum.