English words of three syllables with the stress accent not on the middle one, and words of four or more syllables, generally have a secondary stress accent as well as a primary one. A word such as responsibility can even have two secondary stress accents: res - PON - si - BIL - i - TY. As such it readily fits into iambic meter.
That said, iambic pentameter can and will suffer for being too regular—it becomes hypnotic. Thus judicious or instinctual use of trochaic, pyrrhic, and spondaic substitutions is not only permissible but essential. Spondaic substitutions ( / / for ˘ /) just raise the volume on what would normally be an unstressed syllable, and pyrrhhic substitutions (˘ ˘) just lower the volume on what would normally be stressed one. A trochaic substitution ( / ˘ for ˘ /) would seem to advance one of the five main beats of the line so that it occurs earlier, but more typically it results in a syllable’s worth of delay before that beat, and a rushing through of the two unstressed syllables after it, so that the five beats stay on a steady rhythm. Thus in Orlando’s closing couplet of Shakespeare’s As You Like It 1.2, the words into and unto are trochaic substitutions, each creating a caesura in front of it to divide the line in accordance with the antithesis being expressed; while the two consecutive unstressed syllables -to the and -to a are rushed through:
Thus must I from the smoke | into the smother,
From tyrant duke, | unto a tyrant brother.
(I once had a director in that play who wanted Orlando to pronounce into and unto as iambs [in-TO / un-TO], thus spoiling a very nice effect.)
In Shakespeare, when all else fails one must consider the possibility that two separate accentuation patterns may have been current for the same word, as is the case now with harassment. Thus in Richard II, line 2.2.161 uses the word revenues in the way that is normal today, with primary accent on the first syllable and secondary on the last (note too the spondaic substitution in the second foot):
The plate, coin, revenues, and movables
But in line 4.1.212, the word must clearly be accented on its middle syllable:
My manors, rents, revenues I forgo;