Dilute-gas Bose-Einstein condensates are an exceptionally versatile testbed for the investigation of novel solitonic structures. While matter-wave solitons in one-and two-component systems have been the focus of intense research efforts, an extension to three components has never been attempted in experiments, to the best of our knowledge. Here, we experimentally demonstrate the existence of robust dark-bright-bright (DBB) and dark-dark-bright (DDB) solitons in a spinor F = 1 condensate. We observe lifetimes on the order of hundreds of milliseconds for these structures. Our theoretical analysis, based on a multiscale expansion method, shows that small-amplitude solitons of these types obey universal long-short wave resonant interaction models, namely Yajima-Oikawa systems. Our experimental and analytical findings are corroborated by direct numerical simulations highlighting the persistence of, e.g., the DBB states, as well as their robust oscillations in the trap.PACS numbers: 03.75. Mn, 03.75.Lm Solitons are localized waves propagating undistorted in nonlinear dispersive media. They play a key role in numerous physical contexts [1]. Among the various systems that support solitons, dilute-gas Bose-Einstein condensates (BECs) [2,3] provide a particularly versatile testbed for the investigation of solitonic structures [4][5][6]. In single-component BECs, solitons have been observed either as robust localized pulses (bright solitons) [7][8][9][10][11] or density dips in a background matter wave (dark solitons) [12][13][14][15][16][17][18][19][20][21], typically in BECs with attractive or repulsive interatomic interactions, respectively. Extending such studies to two-component BECs has led to rich additional dynamics. Solitons have been observed in binary mixtures of different spin states of the same atomic species, so-called pseudo-spinor BECs [22,23]. In particular, darkbright (DB) [24][25][26][27][28], and related SO(2) rotated states in the form of dark-dark solitons [29,30], have experimentally been created in binary 87 Rb BECs. Interestingly, although such BEC mixtures feature repulsive intra-and inter-component interactions, bright solitons do emerge due to an effective potential well created by the dark soliton through the intercomponent interaction [31]. Such mixed soliton states have been proposed for potential applications. Indeed, in the context of optics where these structures were pioneered [32,33], the dark soliton component was proposed to act as an adjustable waveguide for weak bright solitons [34]. In multicomponent BECs, compound solitons of the mixed type could also be used for all-matter-wave waveguiding, with the dark soliton building an effective conduit for the bright one, similar to all-optical waveguiding in optics [35]. Apart from pseudospinor BECs, such mixed soliton states have also been predicted to occur in genuinely spinorial BECs, composed of different Zeeman sub-levels of the same hyperfine state [36][37][38]. Indeed, pertinent works [39,40] have studied the existence and dynamic...