Oh ok, and is there a list of spawnable automod somewhere?

Don't know

but you can only get plain white +3s for lvl 18.

https://forums.d2jsp.org/topic.php?t=67041534&f=87

most of the discussion you see around this stuff is going to be talking about oak sage helms. amazon bows, spears, and javs are a little different since they have +tab skills as staffmods. when the amazon item is white, you can get up to +3 tab (bow/xbow or jav/spear, depending on item type) at its minimum level requirement for the item, not the staffmod. when an amazon item is magic or rare, then it uses the applicable magic item tables to determine what +tab skills it can get with the rest of its affixes. meaning, lvl 15 requirement for +1 tab, lvl 30 requirement for +2 tab, and lvl 45 requirement for +3tab, and +3 is also only available on magic items, no rares. the staffmod and affix are only tied together by level requriement, so you can still get a +4 tab at level 30 from getting +2 from staffmods and +2 from affixes.