I would first try something like the following:
<split id="split1" task-executor="taskExecutor">
<flow>
<split next="split2">
<flow>
<step id="step1" parent="s1" next="step2"/>
<step id="step2" parent="s2"/>
</flow>
<flow>
<step id="step3" parent="s3"/>
</flow>
</split>
<split id="split2" task-executor="taskExecutor">
<flow>
<step id="step6" parent="s6"/>
<step id="step7" parent="s7"/>
</flow>
<flow>
<step id="step8" parent="s8"/>
</flow>
</split>
</flow>
<flow>
<split next="split3" task-executor="taskExecutor">
<flow>
<step id="step4" parent="s4"/>
</flow>
<flow>
<step id="step5" parent="s5"/>
</flow>
</split>
<split id="split3" task-executor="taskExecutor">
<flow>
<step id="step9" parent="s9"/>
<step id="step10" parent="s10"/>
</flow>
<flow>
<step id="step11" parent="s11"/>
</flow>
</split>
</flow>
</split>
but making sure the Spring Batch version is after 2.1.5.