By means of the powerful Lanczos algorithm, the nonempirical valence bond (VB) model has been applied to investigate the low-lying electronic states for three typical types of homologous pi-conjugated di- and triradicals. For small radicals including trimethylenemethane, cyclopentadienyltrimethylenemethane, m-benzoquinodimethane, and trimethylenebenzene, our VB results are in remarkable agreement with those obtained from previous sophisticated ab initio molecular orbital calculations. By using the energy difference between the high-spin ground and first excited states for these molecules, the strength of ferromagnetic coupling in these diradicals and their homologous triradicals has been discussed in detail. Our results reveal that ferromagnetic coupling in diradicals TMM and BQDM will decrease to different extent in their homologous triradicals. However, ferromagnetic coupling strength between two "unpaired" electrons in DCPP is found to be 9 times larger than that in its component diradical CPTMM.