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We study the association between active galactic nuclei (AGNs) and environment at scales of 0.01-1 h^–1 Mpc in the IllustrisTNG simulated universe (TNG100). We identify supermassive black hole (BH) pairs and multiples within scales of 0.01, 0.1, and 1 h^–1 Mpc and examine their AGN activity in relation to randomly selected pairs and multiples. The number density of BHs in TNG100 is n = 0.06 h³ Mpc^–3 at z <= 1.5 (n = 0.02 h³ Mpc^–3 at z = 3). About ∼10% and ∼1% of these BHs live in pairs and multiples, respectively, within 0.1 h^–1 Mpc scales. These systems have enhanced likelihood (up to factors of 3-6) of containing high Eddington ratio (η >= 0.7) AGNs compared to random pairs and multiples. Conversely, the likelihood of an AGN to live in 0.1 h^–1 Mpc scale BH systems is also higher (by factors ∼4 for η >= 0.7) compared to random pairs and multiples. ∼10% of ultra-hard X-ray selected AGNs in TNG100 have detectable 2-10 keV AGN companions on 0.1 h^–1 Mpc scales, consistent with observations. On larger spatial scales (∼1 h^–1 Mpc), however, no significant enhancements in AGN activity exist, even at high Eddington ratios. This implies that small-scale (<=0.1 h^–1 Mpc) AGN enhancement is likely driven by galaxy interactions and mergers. Nonetheless, the overall percentage of AGNs living in <=0.1 h^–1 Mpc scale multiples is still subdominant (<=40%). Furthermore, the associated Eddington ratio enhancements of BH systems (as well as merging BHs) is only up to factors of ∼2-3. Our results support the existence of merger-AGN connection in TNG100. However, it plays a relatively minor role in fueling the AGN population.