2014 Citroen C1 drops out of third gear

Check gear shifter cables but if only the one gear affected more likely to be internal with 3/4 shift fork and/or wear of the 3/4 synchronizer / hub assembly

Many thanks could low or contaminated oil have the same affect

Bit doubtful , but certainly worth changing as I've had it work on different cars with similar fault over the years

Many thanks