Can I pay 3 mana for my {Glimpse the Unthinkable} in order to use {Boseiju who shelters All} on it?
I agree with Taysbe on this one. This is the closest rule I could find regarding the question
601.2g (http://imtgapp.com/forum/index.php?action=imtg;area=rule;number=601.2g): The player pays the total cost in any order. Partial payments are not allowed. Unpayable costs can't be paid.
Example: You cast Altar's Reap, which costs {1}{B} and has an additional cost of sacrificing a creature. You sacrifice Thunderscape Familiar, whose effect makes your black spells cost {1} less to cast. Because a spell's total cost is "locked in" before payments are actually made, you pay {B}, not {1}{B}, even though you're sacrificing the Familiar.
Basically, you can't pay more than the casting cost for the spell, and because the spell doesn't have a colorless mana in the cost, you can't use the lands ability to make your spell uncounterable. If an effect would allow you to spend colorless mana as one of the colors of your spell, then you could use the land for this effect.
Also, note, that if an effect increases the cost of the spell ({Thalia}, {Thorn of Amethyst}, {Triniphere}), you would be able to pay that addition with Boseiju and make it uncounterable.