Fighter 3/Monk 3's UAB would be: +5
OA, page 33-34
"Character who have levels of only monk classes always get more unarmed attacks than they would noramlly get based on their base attack bonus (additionaly attacks at intervals of -3, instead of -5). Characters who have levels in other classes as well may or may not gain an advantage from this, and any advantage is often slight.
"Table 3-1: Multiple Unarmed Attacks
Base Attack Bonus Additional Unarmed Attacks
+0 .... ---
+1 .... ---
+2 .... ---
+3 .... ---
+4 .... +1
+5 .... +2
+6 .... +3
+7 .... +4/+1
+8 .... +5/+2
+9 .... +6/+3
+10 ... +7/+4/+1
+11 ... +8/+5/+2
+12 ... +9/+6/+3
+13 ... +10/+7/+4/+1
+14 ... +11/+8/+5/+2
+15 ... +12/+9/+6/+3
"Example: Asako Turima is a 10th-level monk/4th-level henshin mystic. His base attack bonus is +10 (+7 from his monk levels, +3 from his henshin mystic levels). Normally, this would give him an additional attack at +5. He can instead take the additional unarmed attacks listed for his +10 attack bonus (since the entire bonus is derived from monk and mystic levels) on Table 3-1: Multiple Unarmed Attacks, +7/+4/+1. Thus, he can make four attacks in a round with an unarmed strike (or a monk weaopn), at +10, +7, +4 and +1. When using another weapon, he can make two attacks at +10 and +5. He has the unarmed damage, AC bonus and speed of a 14th-level monk.
"Togashi Tidaiko, however, is a 3rd-level samurai/5th-level monk/ 7th-level tattooed monk. Her base attack bonus is +11 (+3 from her samurai levels, +3 from her monk levels, and +5 from her tattooed monk levels). Normally she would have two additional attacks at +6 and +1. She can instead take the additional unarmed attacks listed for a +8 base attack bonus from her monk and tattooed monk levels on Table 3-1, +5 and +2. She makes three attacks in a round, whether armed or unarmed. If unarmed (or using a monk weapon), she can choose whether to strike at +11/+6/+1 or at +11/+5/+2. Armed with an ordinary weapon, she must attack at +11/+6/+1. She has the unarmed damage, AC bonus, and abase speed of a 12-th level monk."
from the PHB, page 55
"Monk Base Attack Bonus: The monk is a special case because her additional unarmed attacks are better than her base attack bonus would suggest. For a multiclass monk fighting unarmed, the character must either use the additional attacks given for her monk levels (only) or the additional attacks that are standard for her combined base attack bonus, but not both. "
