What Car? Q&A - Why do car makers join forces?
A: Car manufacturers often share components to lower overall production costs. The Subaru Justy, for example, is a rebadged Daihatsu Sirion. The Vauxhall Corsa also shares its platform with Fiat's Grande Punto.
The three cars you're looking at are made in the Czech Republic as part of a joint venture between Toyota and PSA (the company that owns Peugeot and Citroen). The engineering is mostly taken care of by Toyota, while PSA handles purchasing and distribution.
The 1.0-litre petrol engine is made by Toyota, and the 1.4-diesel available in the C1 is a PSA unit. The diesel doesn't really make any sense, though, as the petrol engine is much cheaper and does almost as many miles to the gallon.
The C1 is the cheapest to buy, but the Citroen badge on the front means it won't hold its value quite as well as the Toyota. So, we'd pick the one you like the look of best.
All things considered, the Peugeot should work out the cheapest, and the Toyota marginally the most expensive over three years of ownership.