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I am running a series of trials. Each trial has the following possible outcomes:

Event A - 10%

Event B - 20%

Event C - 70%

However, success is not determined by a single trial, but by a series of trials that produce specific sequences.

An example sequence for 20 trials might be:

ACCCBCCACCBCBCCCCBBC

Success is determined by the following two criteria:

Event A

Event B followed by either Event B or Event A (with or without intervening Event Cs)

In the above example sequence, success occurs 4 times, first by the sequence A, then by the sequence BCCA, then by the sequence BCB, and finally by the sequence BB

How does one determine the probability of success given a sequence produced by n trials? That is, for a large n, what is the expected number of successes?

EDIT: It has been suggested that a Markov process be assumed and move forward accordingly. As I have zero knowledge of Markov, I've done a few hours of research and put together the following. (Most sites I encountered were so far over my head, I had no possibility of understanding them.)

The first step in establishing a Markov Chain is to define probabilities of state transitions. Of course, before state transitions can be established, one first has to identify what the states are.

For my problem, I've concluded there are three states.

N is the neutral state. This is the initial state.

B is the state in which Event B has happened but has not yet terminated in success.

S is the success state.

The following is a table of state transition probabilities:

N -> N = 0.7

N -> B = 0.2

N -> S = 0.1

B -> N = 0.0

B -> B = 0.7

B -> S = 0.3

S -> N = 0.7

S -> B = 0.2

S -> S = 0.1

This yields a transition matrix:
$M = begin{bmatrix}0.7 & 0.2 & 0.1\0.0 & 0.7 & 0.3\0.7 & 0.2 & 0.1end{bmatrix}$

The next step is to produce an initial state matrix. Here I start in the N state.
$x_0 = begin{bmatrix}1\0\0end{bmatrix}$

Multiplying them together yields:
$Mx_0 = begin{bmatrix}0.7 & 0.2 & 0.1\0.0 & 0.7 & 0.3\0.7 & 0.2 & 0.1end{bmatrix}begin{bmatrix}1\0\0end{bmatrix} = begin{bmatrix}0.7\0\0.7end{bmatrix} = x_1$

What does this tell me? Am I on the right path at all? How do I continue?

I think you are on the right track, though I suspect the probabilities after each step should add up to $1$ , perhaps with $$x_0 M = begin{bmatrix}1& 0 &0end{bmatrix}begin{bmatrix}0.7 & 0.2 & 0.1\0.0 & 0.7 & 0.3\0.7 & 0.2 & 0.1end{bmatrix} = begin{bmatrix}0.7 & 0.2 & 0.1end{bmatrix} = x_1$$ and $$x_0 M^2 = x_1 M=begin{bmatrix}0.7 & 0.2 & 0.1end{bmatrix}begin{bmatrix}0.7 & 0.2 & 0.1\0.0 & 0.7 & 0.3\0.7 & 0.2 & 0.1end{bmatrix} = begin{bmatrix}0.56 & 0.3 & 0.14end{bmatrix} = x_2$$

The steady-state distribution for your Markov chain is $begin{bmatrix}0.42& 0.4 & 0.18end{bmatrix}$ so the expected number of successes from a large number $n$ of trials will not be far away from $0.18, n$

But you do not start in the steady state. In fact $begin{bmatrix}0.7 ,,,& 0.2 & 0.1,,,end{bmatrix} - begin{bmatrix}0.42& 0.4 & 0.18end{bmatrix} =begin{bmatrix}0.28&-0.2& -0.08end{bmatrix}$ and $begin{bmatrix}0.56 & 0.3 & 0.14end{bmatrix} - begin{bmatrix}0.42& 0.4 & 0.18end{bmatrix} =begin{bmatrix}0.14&-0.1& -0.04end{bmatrix}$ with the difference from the steady state halving at each step

So the expected number of successes after $n$ trials is $$(0.18 - 0.08) + (0.18 - 0.04) + (0.18 - 0.02) + cdots + (0.18 - 0.08 times 2^{-(n-1)})$$ which is $$0.18n - 0.16 left(1- 2^{-n}right)$$

If you consider the occurences to be i.i.d., then you can assume a Markov process and do according calculations.