## 1 Introduction  
Photonic molecules (PhMs), which are composed of two or more coupled optical microcavities, such as whispering-gallery mode microcavities, Fabry-Perot microcavities, and point-defect microcavities in photonic crystal, have attracted broad attention and undergone intensive investigation since they were introduced [(1; 2; 3)]. An updated version of PhMs, plasmonic molecules (PMs) are metallic nanostructures where the individual plasmon modes interact strongly and show distinct collective behavior [(4; 5)]. Analogous to the phenomena observed in atomic and molecular systems, such as Fano resonances [(6; 7; 8)], slow light [(9)], and electromagnetically induced transparency [(10)], interactions between the plasmon modes and the external perturbations also lead to the above-mentioned phenomena in PM systems. Over the past decades, PMs have attracted wide interest for various applications, such as in molecular biology and biomedicine [(11)]. Recently, [PERSON] et al. used the multiple multipole program to analyze homologous series of single, dimer, and trimer silver spherical nanoparticle PMs [(4; 12; 13)] and showed the changes in plasmonic spectra resulting from modification of the structures with the help of group theory. [PERSON] et al. explored the possibility of broadening the in-plane magnetic response of the metal-insulator-metal dimer [14]. [PERSON] et al. reported that the optical properties of self-assembled clusters of spherical metal-dielectric colloids were controlled by varying the number and position of particles in the cluster [15]. However, even the best conventional plasmonic materials, such as gold and silver, suffered huge ohmic losses in the frequency regime of interest. Also, once the geometry of PMs is determined, the electromagnetic (EM) properties are unable to tune, which becomes a bottleneck to further progress for metal-based PMs [16].  
Recently, graphene, which is composed of a monolayer of carbon, has been widely studied due to its unique advantages in electronics and photonics [17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27]. More specifically, a graphene layer is analogous to a metal sheet that is capable of supporting surface plasmon polariton (SPP) waves [28; 29; 30]. Compared with their counterparts supported by conventional plasmonic metals, graphene-supported plasmons demonstrate relatively low damping loss and high confinement [31]. The properties of the plasmon are modified by its surface conductivity, which is further controlled by the chemical potential (Fermi energy) of graphene [32; 33; 34; 35; 36]. Nevertheless, the chemical potential is adjustable by means of chemical doping or local gate voltage [37]. Thus, the EM properties of PMs would be more diverse and colorful once the conventional metallic materials are replaced by graphene. We have proposed and investigated the EM properties of plasmonic dimers composed of two graphene nanodisks, including wavelength splitting and coupling, i.e., hybridization of the fundamental and higher-order plasmonic modes [38; 39].  
To take full advantage of graphene, and to follow the works of [PERSON] and [PERSON] [4; 12; 13] and exploit their idea, we propose a plasmonic trimer composed of three graphene nanodisks arranged in a triangle, which is much more compact than those composed of conventional metals such as Au and Ag. The properties of plasmonic mode evolution of the nanostructure are studied with gradient lowering of the symmetry. More specifically, frequency tunability is explored without changing the geometry of the nanostructure, which is impossible with conventional metals. Finally, a possible fabrication method is pointed out. The proposed graphene nanostructure might find applications in areas such as chemical sensing, light-matter interaction, etc.