An isoform of fibroblast growth factor receptor-1, -2, or -3, where alternative splicing leads to the expression of the IIIb exon encoding the second half of the third immunoglobulin-like domain, which is located in the extracellular domain of these receptors. This variant is expressed in epithelial lineage cells and has differential ligand binding activity when compared to the IIIa and IIIc forms. [ ]