Is it negatively viewed by PhD programs if I completed my masters in 3 years instead of the standard duration of 2 years? I am doing my masters in Mathematics in Germany and I wanted to switch my major area from Geometry to Probability Theory after my first year, so that after my masters I can get a PhD in Statistics.
This would mean that I would finish my master's degree in 3 years instead of 2. Is that negatively viewed by graduate admissions committees in US / Europe?
Edit 1: The candidate must write their masters thesis in the major area and also get  more than a certain number of credit points in the major area
Edit 2: I might be getting the opportunity to be a TA in the stochastics department if I do take another year. So money might not be ab issue(except maybe in the mid semester break)
A Master's in Mathematics is presumably what is necessary for a PhD in Statistics (check with universities you'll apply to): There's no need to switch. It'll cost time and money. Instead, change the classes you'll take as soon as possible.
Bureaucrats are looking to tick-boxes: They'll presumably be looking for a undergraduate-, postgraduate-degree, or both in Mathematics. They'll likely not consider the classes you took.
A PhD supervisor may be interested in the courses you've taken. Their opinions will vary considerably. Get in touch with some candidate supervisors and ask them.
To a supervisor, a Master's with fifty percent more classes may make you more appealing to a candidate with fewer classes. But, it isn't necessary, so I'd suggest you don't give-up a year. You can learn whatever you don't know during your PhD.