I want to have a card which has 3 rfid chips in LF, HF & UHF. Will there be any problems in reading the cards?
I wouldn't expect big problems but it depends a lot on how you want to design this system and what components you will use.
What do you want to achieve? Why do you need 3 frequencies on one card?
Because I use RFID for a number of different applications. e.g to get into my building at work, to unlock lock at home and I have some planned for the future. I want one master card that can work with receivers regardless of what frequency it uses.
You're aware that you don't need different frequencies for this purpose, aren't you? You can just install a reader with the same frequency as you have in the company, read out the ID and just open if it's your card.
How do you plan to produce such a card? I don't know any supplier of ID cards with different RFID technologies in one card.
Yes but there are some applications which might come up in the future where a company or somebody uses a different frequency and I want them to be able to read my card.
I'm actually thinking about integrating it into an iPhone case.
As I already wrote, I don't see a problem having multiple RFID tags of different frequencies in one tag holder (which might be your iPhone case). You might have problems arranging the different antennas so they don't interfere with each other but without more information about your desired application I can only tell you: theoretically it's possible, depending on the practical implementation you might get problems.