Is okay to reject the job offer that they will give me after my probationary period? What negative impacts will it have on me if I do?
If you're through a 3rd party Recruiter:
In the second to the last week of your 3 months, make sure you let your recruiter (not the company) know that you plan to resign from the assignment. And why (that you want to explore different cultures, companies and industries before committing to a permanent position). Your recruiter will appreciate not being caught off guard and can quietly ramp up a replacement before the company knows of your decision. This ensures your relationship with the recruiter is solid and they will be much more open to placing you somewhere else.
If you're on contract directly with the company:
As a general rule, you do not want to tell your employer ahead of time that you're probably only going to work there for 3 months. Training and ramping someone up is costly to a company and if they know in advance that you have a strong desire to move on, they may elect to go with a candidate who is interested in staying for the long term.
You are trying them out just as much as they are trying you out.
Use the the 3 months to get a feel for their culture, the growth opportunity available if you were to stay and whether or not the assignments are challenging, etc. You might be pleasantly surprised at the end of 3 months and want to work there longer. A lot can happen in 3 months.
The bottom line is this: you won't have negative repercussions if you're professional and address your decision at the right time and with honest input as to why you choose to move on. In your case, you want the experience of this 3 month job: it fits your schedule, you're open to evaluating the company just as much as they are evaluating you; but, at the end of that 3 months, you express to them your appreciation for the opportunity and your desire to expand your skills and explore different industries early in your career. You want to have enough diversity early on before you decide where you are going to be long-term. "You don't know what you don't know", as they say, and the only way to know the best path for you is to explore many paths. Companies get this. This attitude will keep the door open should you ever want to work for them again.
Question: What's Your Hurry?
You're just starting your career...you have 30+ years to explore different companies. Unless you don't find this position a fit, why wouldn't you consider staying for a least a year? 3 months here, 6 months there...this can hurt you long term, so switch jobs and companies with the big picture in mind, and a lot can happen in a year. Do you live where you are obligated to stay for X number of years once you accept the permanent position? Just curious.
Your Career is Like a Financial Portfolio - Choose Wisely
Many professionals I meet that are a few years into their career never really planned out the kind of roles they should focus on and the companies they should work for that will elevate their professional status. They jump from company to company, job to job. But as a new professional just hitting the workforce, think of your career path like a financial portfolio. You want each opportunity to build on the previous position, offer new challenges, new skills to learn. You also want to work for reputable and even better, high profile companies. Working for a 7 person company isn't nearly as impressive as working for a 5,000 person company, for example (at least in the US it's like that). Hiring managers and recruiters look at your career path choices as an example of your decision making and strategic long term planning skills. It's not the only thing they use to evaluate you, but it is one of them.
Start With The End In Mind
Start with thinking about where you want to be in your career in 5 years, 10 years, etc. What skills, roles and industries do you need to focus on to get you there? For example: I have a lot of professionals who have devoted the first 10 to 15 years of their career working in the TV/Broadcast industry. Many viewed this profession as exciting and high profile. However, it's a tough industry once you get into your late 30's, settle down and have family. The hours are brutal. Many want to "break into corporate" for the stability and 9-5 schedule but unfortunately, corporate doesn't value their experience nearly as much as they do a professional who comes from another corporation. Their hindsight is usually wishing they had diversified their experience early on before picking one direction to stay in for 10 to 15 years.
Best of luck and I hope you have a prosperous journey!