I feel like "I can not see 3 on a donkey". Olé!
This expression appears to date from the early 20th century ...
"Origen
A principios del siglo XX, en zonas del campo de Andalucía, hubo una campaña para detectar problemas de vista y ayudar a las personas más pobres a solucionar su problema. Estas personas eran casi todas analfabetas, no sabían ni leer ni escribir. Por esta razón, los oftalmólogos no podían utilizar las imágenes tradicionales con letras para medir las dioptrías. En su lugar utilizaban imágenes, dibujos y fotografías que las personas sí sabían interpretar. Una de ellas era la de tres personas montadas en un burro.
At the beginning of the 20th century, in areas of the Andalusian countryside, there was a campaign to detect sight problems and help the poorest people solve their problem. These people were almost all illiterate. For this reason, ophthalmologists could not use traditional images with letters to measure diopters. Instead they used images, drawings and photographs that people knew how to interpret. One of them was that of three people riding a donkey.
Significado
La expresión “no ver tres en un burro” la usamos cuando una persona no ve absolutamente nada. Una expresión con el mismo significado es “ver menos que Pepe Leches” o “estar más ciego que un murciélago / topo”.
The expression, "can't see three people on a donkey" was used when people see nothing. It is an expression with the same significance as, "see less than Papa Leches" or ...
... "be blinder than a bat/mole"
Courtesy Academia Andalusia
Although the print is one hundred years older than the Spanish expression and has a somewhat different meaning - there are only six characters in the picture yet the text refers to seven ... the explanation can be found on The British Museum website for anyone interested.
"Curator's comments, The visual joke "We are Seven", in which the viewer makes up the number of foolish persons"
A clown holding a pig stands beside a woman riding astride an ass with a monkey on her shoulders and an owl perched on her left hand Hand coloured etching
www.britishmuseum.org