3 syllables:
deaconess, reasonless, heedlessness, evenness, sleeplessness, treasonous, peacefulness, feebleness, aesculus, benelux, blessedness, credulous, daedalus, delegates, desmanthus, detritus, dreadfulness, edifice, elegance, elements, elephants, eloquence, eminence, emphasis, emphasise, emulous, endlessness, estimates, evidence, excellence, exodus, fecklessness, fretfulness, friendlessness, gempylus, genesis, gentleness, haemanthus, healthfulness, helpfulness, helplessness, jejuneness, membranous, mespilus, methodists, nebulous, negligence, pegasus, pendulous, penstemon deustus, pestilence, petulance, pleasantness, precedence, precedents, precipice, prevalence, recklessness, regulus, relevance, resinous, resonance, restfulness, restlessness, sedulous, senselessness, sentiments, settlements, sexlessness, specialists, specialness, temperateness, tendinous, tenebrous, tenuous, tetanus, tremulous, vengefulness, venomous, zestfulness

4 syllables:
deceitfulness, ingeniousness, deviousness, acceptableness, adenosis, agenesis, antecedence, beneficence, benevolence, cnicus, contentiousness, defencelessness, defenselessness, developments, digenesis, directionless, distressfulness, dysgenesis, essentialness, forgetfulness, good-temperedness, impetuousness, incessantness, incredulous, inelegance, intelligence, irrelevance, licentiousness, malevolence, momentousness, neglectfulness, oogenesis, persepolis, pink elephants, preeminence, pretentiousness, re-emphasise, relentlessness, respectfulness, state's evidence, successfulness, tendentiousness, unhealthfulness, unhelpfulness, unpleasantness

5 syllables:
agreeableness, acrocephalus, ambidextrousness, androgenesis, anencephalous, arctocephalus, autogenesis, biogenesis, blastogenesis, cacogenesis, caenogenesis, cainogenesis, cenogenesis, chirocephalus, cogent evidence, conscientiousness, cynocephalus, cytogenesis, direct evidence, epigenesis, epinephelus, gynogenesis, hearsay evidence, hydrocephalus, kainogenesis, kenogenesis, leptocephalus, macrocephalous, metagenesis, microcephalous, microcephalus, monogenesis, moral excellence, myxocephalus, ontogenesis, overemphasis, palingenesis, pathogenesis, phylogenesis, pyrocephalus, rule of evidence, sacrilegiousness, sarcocephalus, stenopelmatus, unacceptableness, unpretentiousness, xenogenesis

6 syllables:
disagreeableness, abiogenesis, adult intelligence, agamogenesis, amelogenesis, anthropogenesis, atherogenesis, combat intelligence, current intelligence, funeral-residence, gorgonocephalus, heterocephalus, heterogenesis, indirect evidence, magnetic resonance, nuclear resonance, parthenogenesis, platypoecilus, showing intelligence, signals intelligence, spermatogenesis, terrain intelligence, unconscientiousness, verbal intelligence

7 syllables:
borderline intelligence, circumstantial evidence, nonverbal intelligence, strategic intelligence, tactical intelligence

8 syllables:
artificial intelligence, electronics intelligence, proton magnetic resonance, security intelligence, showing lack of intelligence, telemetry intelligence

9 syllables:
communications intelligence