OCL device code - replaced float3* by float*

The host code allocates device buffers of type float3 for (number of
elements * sizeof(float) * 3) bytes, but sizeof(float3) is 4x4 = 16
bytes.
As a consequence, float3* buffers have been replaced by float* buffers
and the access to x, y, z components is done by correctly computing the
index.
x -> new_index = 3 * old_index + 0
y -> new_index = 3 * old_index + 1
z -> new_index = 3 * old_index + 2