The micromechanics of 3-D braided preform by 3 $ imes$1 pattern has been analyzed Based on the actual yarn paths and interlacing patterns, geometric relations between the structural parameters are analytically established. Analysis of the yarn paths during one repeat of the preform gives the number of different groups of the yarn with the same pattern in the structure which will in turn decide the yarn types used in the formation of hybrid composites where several different types of yarns are used. Observation of the preform surface cut at an angle of 18$^{circ}$gives the interlacing pattern of the yarns and the critical condition of jamming in all directions has been determined with the critical orientation angle of 54$^{circ}$which is smaller than that 1 $ imes$1 pattern preform which is calculated as 60$^{circ}$. This difference in critical orientation angle will result in difference in physical properties of the composites. The maximum yarn volume fraction was determind to be 63.4% which is smaller than 78.5% of 1 $ imes$1 pattern preform. The validity of the analytical model was examined by testing the preforms braided with several different high performance fibers.