Paul1986 wrote:

Hello everyone
Is it possible to "transplant" the carriage of a selectric 1 onto a selectric 3?

It is possible but probably not practical.  Many improvements were made through the iterations of the Selectric.  Using the much older carrier will likely require you to replace the return cable as the end connections will be different in order to work with the carrier.  I know of people who have replaced the Selectric III 210 bicycle ribbon carrier with the Selectric II Selective Ribbon System carrier.