翻译成英语：客户可在这三种平台组合长度中任选其一

There are three ways to assemble with different length avaiable for clients

我翻译的意思是："有三种不同长度的组合方法供客户选用"，我不知道你的平台指的是什么东西，所以避开了这个东西，因为在上文肯定有提及，应该没什么影响

The customers are able to choose desired length from one of the three platform combinations.

The customer is let choose one in may constitute the length in this three kinds platform