I am trying to create an OpenGL ES 3.0 context using GLX. But, when trying to create an OpenGL ES context with glXCreateContextAttribsARB the call fails if I request any version other that 2.0.

I think this part of the mesa code is responsible (src/glx/dri_common.c):
   /* The GLX_EXT_create_context_es2_profile spec says:
    *
    *     "... If the version requested is 2.0, and the
    *     GLX_CONTEXT_ES2_PROFILE_BIT_EXT bit is set in the
    *     GLX_CONTEXT_PROFILE_MASK_ARB attribute (see below), then the context
    *     returned will implement OpenGL ES 2.0. This is the only way in which
    *     an implementation may request an OpenGL ES 2.0 context."
    */
   if (*api == __DRI_API_GLES2 && (*major_ver != 2 || *minor_ver != 0)) {
      *error = __DRI_CTX_ERROR_BAD_API;
      return false;
   }

The code quotes the GLX_EXT_create_context_es2_profile spec but that quote is NOT in the spec (maybe it has been in the past).

The mentioned spec however says that:

... If the version requested is a valid and supported OpenGL-ES version,
    and the GLX_CONTEXT_ES_PROFILE_BIT_EXT bit is set in the
    GLX_CONTEXT_PROFILE_MASK_ARB attribute (see below), then the context
    returned will implement the OpenGL ES version requested.

I interpret this as "If OpenGL ES 3.0 is requested and supported, a context of version 3.0 is returned". Since OpenGL ES 3.0 is supported, I expect a call requesting that version to succeed.

Please tell if you want me to provide more information.

Best regards,
Gustaf