Of course to answer this question properly one would need to understand what is meant by the three terms - space, time & matter - in Spinozas time. Given that Spinoza was influenced by Descarte who himself had been influenced by the success of the new mechanics as exemplified by Newton & Galileo - perhaps the correct place to start are the assumptions made there as exemplified by Newtonian Mechanics, briefly, that space is everywhere the same and that times passes in a homogenous manner everywhere.
how does one use Spinozas system to clarify the status of what time & space is?
One might say for example, that extension itself is the very essence of space.
If matter and space share this essence, could this mean that they are different manifestations of a one unique essence? That although time passes for us in an immediate sense, the character of time in Newtonian Mechanics is the line (suitably understood), so again its essence is extension.
This at least is consistent with the standard view that Spinoza gave God an infinity of essences with only two of the essences being cognisable - extenson and thought - through their modes - with ideas a mode of thought, and matter a mode of extension. In this interpretation, it seems to hold, that time and space cannot be modes of thought, thus they are modes of extension.
So, although matter is usually given in expositions of Spinozas thought as a mode of extension; it seems to follow from his geometrical method that both time & space are too.
This doesn't mean to say that Spinoza has anticipated Einsteins theory, it is an exposition of Newtonian ideas of time & space in Spinozoan terms. One could argue, though, if the essence of matter, time & space is one and the same; then perhaps it could be possible to change one into the other.
This is in fact part of Einsteins achievement. He showed that time & space are interconvertible in quantitative terms; matter wasn't. Lord kelvin suggested that matter could be understood geometrically as a knot - that is as a pure extension. In fact, the main line of both mathematical and physical thinking, in the western tradition, at least, is to reduce phenomena to geometry - that is extension.