Two boards can have the same number of items and still be very different in strength depending on who holds them and what those items do. A carry with three near-BiS items can sometimes produce roughly four times the power of the same unit without items. This is only a rough teaching model, not a universal multiplier.
Start with two questions:
Who should receive the items?
If you cannot make BiS, how far can you compromise?
Item power does not need to be distributed evenly.
Three items on three different units can be much weaker than concentrating those items on the unit responsible for dealing most of your damage.
That creates clear roles:
the carry deals damage
→ the frontline buys time for the carry
First identify your main carry, then concentrate the items that let it perform that role.
If you cannot make the ideal item, do not look only at the item name.
Ask:
What function does this BiS item provide to the carry?
If a carry needs to cast repeatedly, the ideal mana item may not be available. Another mana item may still preserve enough spell frequency to do the job.
Sometimes attack speed can indirectly solve part of the same problem by creating more attacks and therefore more mana.
It is not identical to BiS.
The question is:
How much of the required function can another item preserve?