The aggregation, arrangement and bonding together of proteins on a type 1 RNA polymerase III (Pol III) promoter DNA to form the transcriptional preinitiation complex (PIC), the formation of which is the first step of transcription. Type 1 promoters require TFIIIA for promoter recognition, which then recruits TFIIIC. TFIIIC in turn recruits TFIIIB, which recruits Pol III to the promoter.