Years ago I bought a triple SIM card holder to run on my old Nokia 6310i. I found an outfit on the web and ordered online and it was less than GBP10 IIRC. (I have checked and I had bookmarked their site but it doesn't seem to open now!).
It fitted between the battery and the phone, and had a little switch on the back which changed the SIM card you chose to use. Obviously it only worked one SIM card at a time.
The downsides were it made the 'phone a little bulkier, and you had to turn the 'phone off before you could select another SIM card, but hey, it was easier than running around with three mobile 'phones!