how to merge multiple .fq file?

[renmiaozhen]

I have three pairs .fq file, such as A_1.fq, A_2.fa, B_1.fa, B_2.fq, C_1.fq, C_2.fa.
The three pairs fq file was generated by three times 350bp library construction.
how do I generate accurate k-mer result by FastK?
Look forward to your reply, thank you! @thegenemyers