I took chem HL and honestly I thought it was way easier than my bio HL class, and a lot of my other classes. I think it's one of those classes that people either hate or love. I've never seen anyone in between. That's why it's known as a gruelling class. So I'd suggest trying it out and see which area you fall into.
However, I biggest suggestion would be taking both HL econ and HL chemistry to start off with. If your school allows this, you can have an easier feel for the classes and make a better judgement for your future. For example, if you end up hating chemistry because of HL chem, it may be useful for you to re-consider your specialisation to make yourself happier in the long run.
I know I did this with taking both pscyh HL and maths HL when I started, despite wanting to go into psychology. I dropped psychology halfway through the year. I hated my psychology class and am now incredibly happy to be getting ready to start studying Natural Sciences at uni next year.
Also, what do you mean by "top unis." If it's a place like Cambridge, then you'd be going into Natural Sciences where it is beneficial to have 3 science HLs as it widens your options for the modules you can take, which is important for specialisations. If it's imperial, who have a molecular psychics course, they require you to take HL maths, psychics, and chemistry, and achieve a 7 in chemistry. The required classes are non-negotiable and you will be rejected if you don't take them, as many of my friends have been.