There are several different specifics that can be used for 'additive' resists. First, let's suppose that all resists are either present or not on an object or temporary source (like now) but that having multiple copies of a resist improves your resistance. There are a few obvious approaches:
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(1): 66.7%, 88.9%, 96.3%, ... (2/3, 8/9, 26/27, ...)
(2): 50.0%, 75.0%, 87.5%, ... (1/2, 3/4, 7/8, ...)
(3): 50.0%, 66.7%, 75.0%, ... (1/2, 2/3, 3/4, ...)
(1) is closest to the current system, and continually divides the damage by 3. This currently occurs with permanent + temporary resistance, and this would simply extend this to incorporate multiple permanent sources. (2) is similar, but halves the damage instead of dividing by 3. This is perhaps better balanced with current resistance distribution on items. (3) is weaker again, but quite elegant.
Alternatively, some systems have different (numerical) bonuses to resistance. For example an item might give +40% resistance to fire and another might give +20% resistance to fire. There are three natural ways for this to combine in an 'additive' way:
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(4): add the numbers, if it is 100% or greater, you are immune
(5): add the numbers with a cut-off maximum resistance such as 90%
(6): 'co-multiplication'
(4) and (5) are fairly self-explanatory. (5) features in Diablo.
(6) is more tricky, but features in at least one variant (FA?). You take the complements of the resistances (the complement of X% is (100-X)%) and then multiply these and then take the complement of that. This is also the way that you 'add' probabilities to get the probability of (A or B) from the probability of A and the probability of B, and often comes up in mathematics (e.g. in De Morgan's laws).
For example, if you have +40% resistance from one source and +20% from another, the complements are +60% and +80%. Multiplying these gives +48%, and the complement of this is +52%, so you have +52% resistance.
Note that (6) is the equivalent to (1) and (2). For example, if you have two items with 50% resistance, it gives you 75% resistance, and if you have three it gives you 87.5% resistance. Two items with 66.7% resistance give you 88.9% resistance etc.
I think all six options are workable, but people might want to refer to this list when thinking about them as some of them have problems that others do not. For example (1) makes the game strictly easier, while the others do not.