No to wearing 3 boots of speed, as you can only have one of any item.

Yes, to item effects stacking. Please note that +% items are of base speed/ability/etc, not total. Ie, 2 +10% items, give +20%, not +21% (It doesn't do it this way: First item giving +10%, to 110%, second giving +10% of that, or 121%)

Yes, to BoS and Journeyman Treads. Which should give +10% and +15% If I remember correctly, for +25% of base.

Anything additive, is just that, for example weapon damage.

Hope this helps.