This builds on CreateSean's answer, since you're using Stash. You'll need a math plugin and in this example I'm using Math from Caddis.
{!--set main multi-channel entry list--}
{exp:stash:set_list
name="one"
parse_tags="yes"
}
{exp:channel:entries
channel="ONE_CHANNEL|ANOTHER_CHANNEL|ETC"
dynamic="no"
}
{stash:entry_id}{entry_id}{/stash:entry_id}
{stash:title}{title}{/stash:title}
{/exp:channel:entries}
{/exp:stash:set_list}
{!--set tweet list--}
{exp:stash:set_list
name="two"
parse_tags="yes"
parse_depth="2" {!--run 2x to parse the math plugin--}
}
{exp:channel:entries
channel="TWEET_CHANNEL"
dynamic="no"
}
{stash:entry_id}{entry_id}{/stash:entry_id}
{stash:title}{title}{/stash:title}
{!--set a number to match every third count where the tweet should be output--}
{stash:two_count}{exp:math formula="{count} * 3"}{/stash:two_count}
{/exp:channel:entries}
{/exp:stash:set_list}
{!--output multi-channel list and tweet list--}
{exp:stash:get_list
name="one"
prefix="st1"
}
{!--output tweet on every third loop--}
{if st1:count % 3 == 0}
{exp:stash:get_list:nested
name="two"
limit="1"
match="#{st1:count}#"
against="two_count"
prefix="st2"
}
<li>{st2:entry_id} : {st2:title}</li>
{/exp:stash:get_list:nested}
{!--otherwise output main channel entries--}
{if:else}
<li>{st1:entry_id} : {st1:title}</li>
{/if}
{/exp:stash:get_list}
