It's your dream institution and your dream project, but this is just a patina over the work and the working environment you will be living in the next 3 years. And this patina is unfortunately not that relevant, in the long term.
If you are interested in the academic career, the postdoc is much less relevant with respect to what you do as research than the "management" aspects (i.e. applying for funds, delivering the bureaucracy on time, supervising younger guns, etc...).
My advice is to think of the probation period as a mutual evaluation period: they are evaluating you, but you are also evaluating your new workplace.
Focus on the first "long-term" deadline (1st year project report? 1st year paper? whatever), work towards that goal, at the same time be yourself, be open, be curious, try to get in touch with the research of your peers in the department .... and then after 3 months make your decision.
I think you will need 3-4 months to scratch the patina of your new working environment. If, after three months you are enjoying the project, the working environment, your work-life balance, the place and the project, it is unlikely you will be performing so badly that you do not pass the probation period. It is very easy to assess if a person is enjoying the work, it is very hard to assess quality of a PostDoc work after 6 months, and no one has the time to do that, so the evaluation will be on your motivation to keep on working on the project for 3 years , rather than on some metrics.
If you are not enjoying it, just focus on yourself and work hard towards getting your own funds for your own project, make your own plan B. If you have no academic interests, use the remaining months to sharpen your CV for your exit to the private world.
 pay attention not to corner yourself in a hole. They hired you for 3 years to work on a certain project, however your CV in 3 years down the road will benefit if you showed some independence in getting funds, supervising students, etcetc ... keep your possibilities open.