The following are known Action Replay Codes for Mega Man Battle Network 3: Blue Version on Nintendo Game Boy Advance (GBA).
These codes were also known as GameShark Codes and in most cases will work on both devices.

These codes are for the U.S. version of the game.

Master Code – Must Be On
27b0cce2
aaaf9985
160dee65
2c1f5955

Infinite HP
67d877dd
4d37c5f6

Max HP
f4f1efc2
c536f678

Infinite Zenny
cf9e65a2
643185eb

Easy Enemies
9008afe1
f8117e44
85748a84
fc2e27dd
fbba951a
8939b0f5

Full Custom Bar
ba7a342f
91de9f39

Full Chip Select
e84c9e88
bac57933

Full Movement
a9afcd65
c82c08ea
004c9023
1c47bf77

Have All Sub Chips
abdcafa9
e6673bdc
44e437c4
3d2bc882

No Random Battles
a0b2ee68
8a25021e

Have All Key Items
1d7bbca3
2123ad4f
c89a41ad
5b7cada0

Max Bug Frag
b4732afa
b145c5b4

Chips Do 9,999 Damage
5d97aa53
0f59216a
b6084f13
01e26fa7

Max Attack
1c09ffda
f98184a9

Max Speed
ba9cd517
1df26ba0

Max Charge
e0d43b47
972fd330

RegChip 99MB
4d07d7c4
8b90287c